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Songbird Diversity and Habitat Use in Response to Burning
on Grazed and Ungrazed Mixed-Grass Prairie
By
Krystle White
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Submitted to the Faculty of Graduate Studies
In Partial Fulfillment of the Requirements
For the Degree of
Master of
Natural Resources
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Natural Resources Institute
University of Manitoba
Winnipeg, Manitoba
August, 2009
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Songbird Diversity and Habitat Use in Response to Burning
on Grazed and Ungrazed Mixed-Grass Prairie
By
Krystle White
A Thesis/Practicum submitted to the Faculty of Graduate Studies of The University of
Manitoba in partial fulfillment of the requirement of the degree
of Master of Natural Resources Management (M.N.R.M)
© 2009
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Abstract
Grassland songbird populations in North America are declining rapidly. The
descending population trends have paralleled the loss of grassland habitat, which is in
part due to habitat degradation from altered ecological processes such as fire suppression.
On the remaining tracts of native grassland, it is therefore important to maintain natural
disturbances and incorporating fire into management plans may represent a key
conservation strategy. However, because cattle and other livestock graze many remaining
areas of the mixed-grass prairie, the interacting effects of burning and grazing must also
be considered if fire is to be integrated into conservation strategies. This study provides
the only burning-grazing interaction assessment on songbird diversity and habitat
structure in the mixed-grass prairies of Canada.
In the summer of 2006 several wildfires occurred within and around the East
Block of Grasslands National Park and the neighbouring Mankota community grazing
pastures in southern Saskatchewan. The fires provided a valuable opportunity to assess
how burning influences the songbird community on both ungrazed and grazed areas of
northern mixed-grass prairie.
The focus of my study was to evaluate how fire influenced the songbird
community and the use of habitat. Specifically, my objectives were to 1) document how
the interaction between burning and grazing influenced songbird abundance and
diversity; 2) determine if the effects of burning and grazing on the songbird community
can be explained by a relationship between the birds and habitat structure; and 3)
determine if the songbird community and habitat structure responses to burning are
unique from their responses to grazing.
i
To accomplish these objectives I surveyed songbirds using five-minute, 100-m
radius point-count plots in four different treatment groups: burned-ungrazed, burnedgrazed, unburned-grazed, and unburned-ungrazed prairie. I gathered habitat structure data
within the point-count plots using vegetation height, visual obstruction, litter depth, and
percent ground cover of litter, standing dead, exposed bare ground, grass, forbs, and
shrubs.
Results suggest a conservation strategy for maintaining high levels of songbird
diversity with burning on ungrazed mixed-grass prairie. On prairie with a light- to midintensity grazing regime, burning did not affect songbird diversity levels. The structural
components of vegetation generally decreased as a result of burning and grazing, yet the
two disturbances generated different levels of litter and exposed bare ground. A positive
relationship of species diversity with burned and grazed vegetation suggests that high
species diversity targets can be achieved with vegetation of low- to mid-densities and
ground cover. While burning increased the species diversity of the songbird community,
it had a detrimental effect on the abundances of Sprague’s pipits (Anthus spragueii) and
Baird’s sparrows (Ammodramus bairdii), indicating the need to maintain habitats in a
variety of successional stages suitable for the variety of grassland songbirds.
ii
Acknowledgements
I would like to extend my gratitude to those individuals whose assistance helped
to make this thesis possible. First of all, I would like to thank my supervisor, Dr. Nicola
Koper, for her guidance on the writing process and statistical analyses that greatly
improved all aspects of this thesis. I would also like to thank the members of my advisory
committee, Dr. Spencer Sealy and Dr. Iain Davidson-Hunt, for generously providing their
time and advice.
I would like to acknowledge Barbara Bleho and Allison Selinger for assisting
with point-count surveys and vegetation measurements, as well as Tim Teetart and
Trinette Konge, who also helped with data collection. I am grateful to Pat Fargey,
Species at Risk/Ecosystem Management Specialist with Grasslands National Park,
Michael Fitzsimmons, Ecosystem Scientist with Grasslands National Park, and John
Wilmshurst, Ecosystem Science Coordinator with Jasper National Park, for providing
valuable insights on the functions and processes of grassland fires within Grasslands
National Park and information on the park management plans.
Funding for this project was provided by Parks Canada. Additional financial
support was received from the Province of Manitoba through the Manitoba Graduate
Scholarship (2008, 2009), the Province of Saskatchewan through the Fish and Wildlife
Development Fund Student Award (2008), and the Sixth Prairie Conservation and Endangered
Species Conference Fellowship (2008).
iii
Table of Contents
Abstract ................................................................................................................................ i
Acknowledgements ............................................................................................................ iii
List of Tables ..................................................................................................................... vi
List of Figures ................................................................................................................... vii
List of Appendices ........................................................................................................... viii
1.0 Introduction ................................................................................................................. 1
1.1 Background ............................................................................................................... 1
1.2 Research Objectives .................................................................................................. 2
1.3 Hypothesis................................................................................................................. 3
1.4 Project Significance .................................................................................................. 4
1.5 Research Limitations ................................................................................................ 7
2.0 Literature Review ....................................................................................................... 9
2.1 Habitat Associations of Common Grassland Songbirds ........................................... 9
2.1.1 Horned Lark ..................................................................................................... 11
2.1.2 Sprague’s Pipit................................................................................................. 12
2.1.3 Clay-colored Sparrow...................................................................................... 13
2.1.4 Vesper Sparrow................................................................................................ 14
2.1.5 Savannah Sparrow ........................................................................................... 14
2.1.6 Baird’s Sparrow ............................................................................................... 16
2.1.7 McCown’s Longspur ........................................................................................ 17
2.1.8 Chestnut-collared Longspur ............................................................................ 18
2.2 Historical Occurrence of Fire and Grazing Disturbances ....................................... 19
2.3 The Role of Fire in Mixed-Grass Prairies ............................................................... 21
2.4 Combined Role of Fire and Grazing on Mixed-Grass Prairie ................................ 24
2.5 Effects of Burning and Grazing on Grassland Songbirds ....................................... 26
3.0 Methods...................................................................................................................... 29
3.1 Study Area .............................................................................................................. 29
3.1.1 Site description................................................................................................. 29
3.1.2 Burning and grazing locations......................................................................... 30
3.1.3 Experimental Design........................................................................................ 30
3.2 Songbird Surveys .................................................................................................... 32
3.2.1 Data Organization ........................................................................................... 35
3.3 Habitat Surveys ....................................................................................................... 37
3.3.1 Data Organization ........................................................................................... 39
iv
3.4 Data Analysis .......................................................................................................... 40
3.4.1 Songbird Community and Habitat Structure ................................................... 40
3.4.2 Habitat Associations for May versus July 2008 .............................................. 42
3.4.3 Year Effect........................................................................................................ 42
4.0 Results ........................................................................................................................ 44
4.1 Songbird Community Relationship with Burning and Grazing .............................. 44
4.1.1 First Year Post Burn (2007) ............................................................................ 44
4.1.2 Second Year Post-Burn (2008) ........................................................................ 48
4.2 Songbird Community Relationship with Habitat Structure .................................... 52
4.2.1 First Year Post-Burn (2007) ............................................................................ 52
4.2.2 Second Year Post-Burn (2008) ........................................................................ 52
4.3 Habitat Structure Relationship with Burning and Grazing ..................................... 54
4.3.1 First Year Post-burn (2007)............................................................................. 54
4.3.2 Second Year Post-Burn (2008) ........................................................................ 56
4.4 Songbird Community Relationship with July 2008 Habitat Structure ................... 59
5.0 Discussion................................................................................................................... 61
5.1 Species Richness and Heterogeneity ...................................................................... 63
5.2 Species Evenness .................................................................................................... 65
5.3 Individual Species Analyses ................................................................................... 66
5.4 Additional Species .................................................................................................. 69
5.5 Habitat Structure ..................................................................................................... 71
5.6 Habitat Associations for May versus July 2008 ..................................................... 73
6.0 Conclusions and Recommendations ........................................................................ 74
Literature Cited .............................................................................................................. 78
v
List of Tables
Table 1. Number of sites and plots in each treatment group .............................................32
Table 2. Predictor variables in each Model used to assess the grassland songbird
community ................................................................................................................41
Table 3. Significant treatment effects (burning main effect, grazing main effect, or
burning-grazing interaction) on the songbird community, June 2007 ......................45
Table 4. Comparison of songbird diversity levels for 1) burned-ungrazed prairie vs.
burned-grazed, unburned-grazed, and unburned-ungrazed, and 2) unburnedungrazed prairie vs. burned-grazed, burned-ungrazed, and unburned-grazed,
June 2007 ..................................................................................................................45
Table 5. Significant treatment effects (burning main effect, grazing main effect, or
burning-grazing interaction) on the songbird community, May – June 2008...........49
Table 6. Comparison of songbird diversity levels for 1) burned-ungrazed prairie vs.
burned-grazed, unburned-grazed, and unburned-ungrazed, and 2) unburnedungrazed prairie vs. burned-grazed, burned-ungrazed, and unburned-grazed,
May – June 2008 .......................................................................................................49
Table 7. Significant songbird community relationships with vegetation structure,
June – July 2007........................................................................................................52
Table 8. Significant songbird community relationships with vegetation structure,
May – June 2008 .......................................................................................................53
Table 9. Significant songbird community relationships with ground cover, May –
June 2008 ..................................................................................................................53
Table 10. Significant treatment effects (burning main effect, grazing main effect, or
burning-grazing interaction) on the vegetation structure, July 2007 ........................54
Table 11. Significant treatment effects (burning main effect, grazing main effect, or
burning-grazing interaction) on the vegetation structure and ground cover,
May 2008 ..................................................................................................................56
Table 12. Significant songbird community relationships with July vegetation
structure, May – July 2008........................................................................................60
Table 13. Significant songbird community relationships with July ground cover,
May – July 2008........................................................................................................60
vi
List of Figures
Figure 1. Study location in the East Block of Grasslands National Park of Canada
and the Mankota Community Pastures in Southern Saskatchewan ..........................29
Figure 2. Clustered study design in southern Saskatchewan ............................................31
Figure 3. Diagram of vegetation subplots for the habitat surveys ....................................39
Figure 4. Average songbird species per plot for species richness, Shannon-Wiener
heterogeneity index (scaled to units of species), and evenness of species
distributions, June 2007 ............................................................................................46
Figure 5. Average number of individuals per plot for horned lark, Sprague’s pipit,
Baird’s sparrow, and chestnut-collared longspur, June 2007 ...................................47
Figure 6. Average number of individuals per plot for the five most common
species, horned lark, Sprague’s pipit, Savannah sparrow, Baird’s sparrow, and
chestnut-collared longspur, June 2007 ......................................................................47
Figure 7. Average songbird species per plot for species richness, Shannon-Wiener
heterogeneity index (scaled to units of species), and evenness of species
distributions, May – June 2008 .................................................................................50
Figure 8. Average number of individuals per plot for horned lark, Sprague’s pipit,
Baird’s sparrow, and chestnut-collared longspur, May – June 2008 ........................51
Figure 9. Average number of individuals per plot for the five most common
species, horned lark, Sprague’s pipit, Savannah sparrow, Baird’s sparrow, and
chestnut-collared longspur, May – June 2008 ..........................................................51
Figure 10. Average vegetation structure measurements per plot for maximum
vegetation height (cm), visual obstruction reading (cm), and litter depth (cm),
July 2007 ...................................................................................................................55
Figure 11. Average vegetation structure measurements per plot for maximum
vegetation height (cm), visual obstruction reading (cm), and litter depth (cm),
May 2008 ..................................................................................................................57
Figure 12. Average percent ground cover per plot for litter cover, standing dead
vegetation cover, exposed bare ground, grass cover, forb cover, and shrub
cover, May 2008 .......................................................................................................58
Figure 13. Comparison of average percent ground cover per plot, May 2008 .................59
vii
List of Appendices
Appendix I. Songbird species included in 2007 data analysis and fly-bys removed........85
Appendix II. Songbird species included in 2008 data analysis and fly-bys removed ......86
Appendix III. Means and standard errors for the songbird community per plot in
the four treatment groups; burned-grazed, burned-ungrazed, unburned-grazed,
and unburned-ungrazed, June 2007 ..........................................................................87
Appendix IV. Means and standard errors for the songbird community per plot in
the four treatment groups; burned-grazed, burned-ungrazed, unburned-grazed,
and unburned-ungrazed, May – June 2008 ...............................................................87
Appendix V. Means and standard errors for the vegetation measurements per plot
in the four treatment groups; burned-grazed, burned-ungrazed, unburnedgrazed, and unburned-ungrazed, July 2007 ..............................................................88
Appendix VI. Means and standard errors for the vegetation measurements per plot
in the four treatment groups; burned-grazed, burned-ungrazed, unburnedgrazed, and unburned-ungrazed, May 2008 ..............................................................88
viii
1.0 Introduction
1.1 Background
It has been well documented that populations of grassland birds are declining
rapidly in North America (Herkert 1995, Brawn et al. 2001, Samson et al. 2004, Brennan
and Kuvlesky 2005, Askins et al. 2007). The descending population trends have
paralleled the loss of grassland habitat (Herkert 1995, Fondell and Ball 2004, Samson et
al. 2004), which has been attributed to habitat fragmentation; land conversions to
cropland, urban, rural, and industrial development; invasions of exotic and woody
species; and soil erosion (Samson and Knopf 1994, Madden et al. 1999, Fritcher et al.
2004, Brennan and Kuvlesky 2005). In addition, areas of remaining prairie habitat have
been degraded by altered ecological processes, including fire suppression (Samson and
Knopf 1994, Risser 1996, Madden et al. 1999, Brawn et al. 2001). Due to the rates of
habitat loss and declining populations of many grassland species, native grasslands are
considered to be the most vulnerable ecosystem in North America (Samson and Knopf
1994), while it is also the least protected (Hoekstra et al. 2005).
To conserve grassland birds, there is a need for appropriate management of the
remaining prairie habitat. However, because of fire suppression, some grasslands are
being converted to later seral stages, which include taller vegetation, greater canopy and
ground cover, greater litter depths, and shrub invasion on some of the more temperate
grasslands (Brawn et al. 2001, Fritcher et al. 2004, Brennan and Kuvlesky 2005). The
transition to later seral stages ultimately degrades the remaining grassland habitat and,
1
therefore, results in a functional loss of habitat for many grassland species due to
successional changes (Samson and Knopf 1994, Madden et al. 1999, Brawn et al. 2001).
On remaining grassland tracts, it is important to maintain the processes of natural
disturbances, such as fire, because they reintroduce early seral vegetation typical of
grassland ecosystems (Lesica and Cooper 1999, Brawn et al. 2001, Fritcher et al. 2004).
Thus, incorporating fire into grassland management may represent a key conservation
strategy for protecting grassland birds and maintaining grassland ecosystems (Lesica and
Cooper 1999, Brawn et al. 2001, Fritcher et al. 2004, Samson et al. 2004). However,
because cattle and other livestock graze many remaining areas of the mixed-grass prairie,
the interacting effects of burning and grazing must also be considered if fire is to be
integrated into conservation strategies. This study is the first burning-grazing interaction
assessment on songbird diversity and habitat structure in the mixed-grass prairie of
Canada.
1.2 Research Objectives
The focus of my study was to evaluate how fire influences the songbird
community and habitat use in the northern mixed-grass prairie of southern Saskatchewan.
The specific objectives of my study were:
1. To document how the interaction between burning and grazing influenced
songbird abundance and diversity.
2. To determine whether the effects of burning and grazing on the songbird
community can be explained by a relationship between the birds and habitat
structure.
2
3. To determine whether the songbird community and habitat structure responses to
burning are unique from their responses to grazing.
1.3 Hypothesis
Fire and grazing disturbances alter the structure of the vegetation and thereby
change the configuration of grassland habitat. Burning results in short and sparse
vegetation that is grass dominated, with low amounts of litter and forbs (Madden et al.
1999). Similar to burning, grazing also typically reduces the height and density of
vegetation and reduces litter (Lesica and Cooper 1998, Biondini et al. 1999). Because
many grassland songbirds use habitat of different vegetation structure (Wiens 1969,
Bollinger 1995, Madden et al. 2000, Brawn et al. 2001), I predict that the different
disturbance treatments will provide suitable habitat for different birds based on their
habitat preferences. Specifically, birds associated with shorter and sparser vegetation and
lower litter depths will prefer burned and grazed prairie to unburned-ungrazed prairie.
Most grassland birds are strongly associated with vegetation of low- to midheights and densities, perhaps because they evolved alongside frequent grassland
disturbances that historically generated early successional vegetation structures (Mengel
1970, Knopf 1996, Brawn et al. 2001). Because post-fire habitats, in both grazed and
ungrazed mixed-grass prairie, produce early seral vegetation, I predicted that burned
prairie will support more songbird species than unburned prairie. Likewise, fewer
songbird species in unburned-ungrazed prairie are predicted due to taller and denser later
seral vegetation that may be less attractive habitat to many grassland species.
3
1.4 Project Significance
Incorporating fire into grassland management may represent a key conservation
strategy. However, natural disturbances like burning and grazing, which are credited with
maintaining prairie ecosystems, must be understood before they can be used as
conservation strategies (Askins et al. 2007). It is beneficial for studies of natural
disturbances to be conducted in large, natural areas of grassland, to reveal their influence
on the ecosystem as close as possible to historic conditions (Askins et al. 2007).
Grasslands National Park of Canada (GNPC), established to protect and maintain a
portion of Canada’s mixed-grass prairie ecosystem (Parks Canada 2002), is a large,
natural area, where such studies can be conducted. The information collected in this study
can be incorporated specifically into management plans for GNPC, and may also be used
to inform conservation of other areas of northern mixed-grass prairie with a similar
moisture regime. Previous studies on the effects of disturbances on grassland
communities have determined that results vary depending on the ecosystem in question
and that place-specific studies are required (Overbeck et al. 2005). In particular, the
effects of fire are highly variable due to climatic conditions, topography, influences of
other natural disturbances, soil type, and moisture regimes (Madden et al. 2000,
Overbeck et al. 2005). The frequency, intensity, season, and scale of burning, as well as
land use history, further influence the effects of fire on grasslands (Collins and Barber
1986, Madden et al. 1999). Because of the spatially dynamic effects of fire, research
conducted in locations with significantly different moisture regimes and growing seasons,
such as temperate grasslands of the United States, are of limited use in informing fire
management strategies in the northern mixed-grass prairie. Therefore, to incorporate
4
burning into GNCP management plans, or into Canadian grassland conservation
strategies in general, there is a need to look at the effects of fire within the northern
mixed-grass prairie of Canada.
Fuhlendorf et al. (2006) suggest that research regarding land conservation
strategies should examine the interactions of natural disturbances because of their
historical importance in the evolution of ecosystems. With regards to fire and grazing in
particular, these interactions are also important for modern prairie conservation. Since
many remaining areas of the mixed-grass prairie are dominated with grazing by cattle and
other livestock (Bragg and Steuter 1996, Risser 1996, Fondell and Ball 2004, Samson et
al. 2004, Brennan and Kuvlesky 2005), the combined effects of burning and grazing
should be considered if fire is to be integrated into grassland management. Despite this
research need, studies of the interacting effects of burning and grazing in mixed-grass
prairie is rare. Previous studies regarding burning-grazing interactions have
predominantly been conducted in tall-grass prairie (e.g., Collins 1987, Vinton et al. 1993,
Zimmerman 1997, Trager et al. 2004, Fuhlendorf et al. 2006, Powell 2006), because of
the naturally high frequency and intensity of fires in temperate grasslands (Wright and
Bailey 1982). However, because of differences in moisture regime and species
composition, effects of burning in tall-grass prairies are probably significantly different to
effects in mixed-grass prairies (Wright and Bailey 1982). I am aware of only two other
studies that have examined burning-grazing interactions in mixed-grass prairie and both
were conducted in the more temperate prairies of North Dakota. In the first study, Kruse
and Bowen (1996) looked at burning-grazing interaction effects on breeding waterfowl
with respect to all four possible treatments: burned-grazed, burned-ungrazed, unburned-
5
grazed, and unburned-ungrazed. In the second study, Danley et al. (2004) looked at the
habitat and songbird community with regards to burned-grazed or burned-ungrazed land
only. In addition, while many studies have examined the habitat associations of grassland
birds (e.g., Wiens 1969, Owens and Myres 1973, Stewart 1975, Pylypec 1991, Madden et
al. 1999, Madden et al. 2000, Danley et al. 2004), these assessments in the northern
mixed-grass prairie are limited (Davis et al. 1999), especially with regards to the effects
of fire (Johnson 1997). Thus, my study contributes new information with respect to
interaction effects of burning and grazing on songbird diversity and habitat structure in
northern mixed-grass prairie.
Studies that examine the effects of fire in grassland ecosystems usually include
assessments of a single burn within each independent category, or within each treatment
type (e.g., Collins and Barber 1986, Collins 1987, Pylypec 1991, Madden et al. 1999,
Harrison et al. 2003). In these studies, replication is restricted to several plots within a
single burn, and is therefore pseudoreplicated (Hurlbert 1984, Quinn and Keough 2002).
In contrast, my study includes five different burns, one that was large enough to place
two sites 6 km apart, and two that overlapped half in grazed and half in ungrazed fields,
for a total of eight different burned sites; four in ungrazed areas, and four in grazed areas.
Whereas the zone of inference for a single burn is of that particular patch of burned
prairie only, multiple burns reflect a much broader zone of inference. The presence of
multiple burns allows for treatment replication and increases the transferability of the
results beyond the single patch level (Quinn and Keough 2002).
6
1.5 Research Limitations
My study reveals the songbird diversity and relative abundance within the
different treatment groups, and the potential relationship with vegetation structure.
Although these parameters reveal habitat use by grassland birds and, therefore, provide
valuable insights into habitat quality (Wiens 1969), they do not address specific measures
of habitat quality like factors such as nest success, survivability, brood parasitism,
predation risk, food quality, or individual body weight. For instance, in a study
examining prairie bird density in relation to nest success, Vickery et al. (1992) showed
that a greater number of individuals within a given area does not necessarily indicate
higher habitat quality and higher reproductive success. Because each species exhibits a
unique reproductive response to density, Vickery et al. (1992) suggest that density should
not be used to determine habitat quality. Nonetheless, parameters obtained from point
counts, like the mean abundance, have demonstrated reliable predictions for reproductive
success and therefore habitat quality, in many cases (Betts et al. 2005). In addition,
determining relationships between birds and vegetation structure may clarify reasons
behind population declines and may be helpful in determining land management plans
and conservation strategies (Davis et al. 1999).
Another limitation is the absence of baseline data from before the grazing and
burning disturbances. Ideally, the songbird and vegetation surveys would be conducted
prior to the treatment, to determine variability in songbird diversity and habitat structure
independent of burning and grazing. This information would then be compared to the
results of surveys conducted after the treatment. However, when working with applied
science and land management techniques, this is not always a possibility. For instance,
7
when examining natural wildfires (as is the case with my study), pre-disturbance
assessments are rare because it is impossible to predict the occurrence and location of
lightning strikes. As a result, comparing disturbed and undisturbed prairie, as opposed to
before and after disturbance, is a common technique used in grassland ecology (e.g.,
Johnson 1997, Biondini et al. 1999, Madden et al. 1999, Danley et al. 2004). In my study
area, unburned-ungrazed prairie occurs in the same vicinity as the grazed and burned
prairie; all sites are in upland areas and consist of comparable mixed-grass prairie
vegetation. Thus, comparing unburned-ungrazed prairie with grazed and burned prairie
provides the best possible natural representation of the response to the different
disturbances.
8
2.0 Literature Review
2.1 Habitat Associations of Common Grassland Songbirds
The selection of habitat by birds can be sensitive to both local and landscape level
factors. Local factors include vegetation structure and composition at nesting or foraging
sites, whereas landscape factors encompass a broader scale and may include habitat
amount and fragmentation. Previous studies have shown that landscape features
sometimes influence grasslands birds, especially in terms of nesting success (see Koper
and Schmiegelow 2006 and studies mentioned within). Although it is important to
understand the influence of landscape features in conservation practices, such as the
value of preserving large areas of native habitat, grassland songbirds may be more
influenced by local factors during the selection of breeding habitat (Koper and
Schmiegelow 2006).
When comparing the influence of local and landscape factors on habitat selection
and nest success of grassland birds, Koper and Schmiegelow (2006) found that patterns in
species abundance and nest success were generally explained more by a relationship with
local factors such as vegetation height, density, litter depth, and percent bare ground, than
by landscape factors such as amount and shape of grassland patches. However, the
influence of spatial scales (i.e., local vs. landscape level habitat characteristics) was
species specific; for example, chestnut-collared longspur (see Appendices I & II for
scientific names of all species) and western meadowlark responded strongly to both local
and landscape features, while horned lark, Sprague’s pipit, vesper sparrow, Savannah
sparrow, and brown-headed cowbird responded strongly only to local habitat
9
characteristics (Koper and Schmiegelow 2006). Fondell and Ball (2004) further suggest
that grassland birds select habitat not only at a local scale, but based specifically on the
vegetation structure at nest sites. In this study, important nest site characteristics, such as
clumps of dense vegetation for shelter, were consistently chosen by many species
irrespective of plot level vegetation structure in grazed versus ungrazed prairie (Fondell
and Ball 2004). Therefore, habitat management at the local scale, by altering vegetation
characteristics, is necessary for maintaining species populations and reproductive
success.
These studies are consistent with others, in which vegetation variables at a fine
scale, such as territory or nest site, were more strongly related to bird abundances than
broader scale characteristics (Wiens 1969, Bollinger 1995, Davis 2005). However,
minimum area requirements must be met before individuals are present in the area and
habitat associations with local vegetation features can be recognized (Davis 2004, Van
Dyke et al. 2007). Once minimum area requirements are met, the structure of the
vegetation becomes important to birds because it is what determines available display
perches, shelter for nests, and suitable foraging areas (Wiens 1969, Wiens 1973). Most
songbirds establish their territories where all facets of breeding (i.e., display, nesting,
foraging) are provided within the boundaries, or at least nearby (e.g., foraging by some
species may be done off-territory), as most species typically do not venture far from their
territories (Wiens 1973). These territories range in size among species and regions. For
instance, territories can range from 0.6 to 1.1 ha for vesper sparrows, 0.2 to 1.4 ha for
Savannah sparrows, and 1.8 to 3.4 ha for western meadowlarks (Wiens 1969).
10
The following is a description of habitat associations by a few of the common
songbirds observed in the study area, including: horned lark, Sprague’s pipit, claycolored sparrow, vesper sparrow, Savannah sparrow, Baird’s sparrow, McCown’s
longspur, and chestnut-collared longspur.
2.1.1 Horned Lark
Horned larks are widespread in North America, but breeding is typically restricted
to open grassland with high components of bare ground and short grasses (Beason 1995,
Davis and Duncan 1999), such as burned or grazed prairie (Cody 1985, Stewart 1975,
Johnson 1997). Non-prairie habitats include deserts, alpine habitat, and tundra (Beason
1995). In addition, horned larks have adapted to hayfields and crop fields where
vegetation is short and/or sparse (Beason and Franks 1974, Stewart 1975).
This species arrives on their breeding range as early as March or April (Stewart
1975) and begins nesting and territorial behaviour earlier than some other songbirds in
this study that typically begin clutch initiation in May (e.g., Sprague’s pipit [Robbins and
Dale 1999], Baird’s sparrow [Green et al. 2002], McCown’s longspur [With 1994]).
However, the peak breeding season for horned larks and all other songbirds in this study
occur during the same time frame of May to July (Stewart 1975, Dickson and Dale 1999).
Territories are multi-functional and include all necessary facets for breeding, such as
courtship, nesting and foraging (Beason 1995). The size of the territory has reportedly
been influenced by habitat structure where population densities increase with amount of
bare ground (Beason 1995). Courtship displays are conducted on the ground, where the
male opens and droops his wings, spreads his tail, and struts in front of the female
11
(Beason 1995). Males predominantly sing from the surface of the ground or in the air
(Beason and Franks 1974), but may use rocks or shrubs for perches when defending their
territories (Beason 1995). Nests are located on the ground, in a shallow depression that is
either natural or made by the female, and are often near some type of shelter such as
clumps of grass, rocks, or cow dung (Beason and Franks 1974, Beason 1995). Like
nesting, foraging also occurs on barren grasslands, where adults feed mostly on seeds
during the breeding season, and feed insects to their young (Beason 1995).
2.1.2 Sprague’s Pipit
Sprague’s pipits are found in native prairies of the northern Great Plains, where
breeding habitat is located in well-drained open grasslands with intermediate vegetation
heights, densities, and litter depths (Owens and Myres 1973, Prescot 1997, Robbins and
Dale 1999). Lightly grazed or ungrazed prairie in southern Saskatchewan reportedly
provides this type of vegetation structure (Stewart 1975, Johnson 1997, Davis et al. 1999,
Madden et al. 2000). This species has suffered severe population declines as a result of
habitat loss, which is primarily due to cultivation, but also other agricultural activities
such as intensive grazing and haying (Prescot 1997). Sprague’s pipits are considered
Threatened under Schedule 1 of the Species at Risk Act.
Territories are multi-functional and include all necessary facets for breeding, such
as courtship, nesting and foraging (Robbins and Dale 1999). Males defend their territories
by performing an aerial display that is also used in courtship. During the aerial display,
the bird flies up to a height of 50-100 m in an undulating pattern and circles over the
territory while singing (Robbins and Dale 1999). Males will rarely sing from the ground
12
or on a song perch (Prescot 1997). Nests are located on the ground and at the base of a
clump of grass with dense surrounding vegetation and little bare ground (Sutter 1997).
Sprague’s pipits have shown an association with standing dead vegetation and residual
vegetation cover (Davis and Duncan 1999), since most nests have a dome or canopy
made from long grasses that grow beside the nest (Sutter 1997, Robbins and Dale 1999).
Sprague’s pipits forage on the ground by walking through tall vegetation and searching
mostly for arthropods on the ground surface or gleaning (picking prey off vegetation)
from grasses (Robbins and Dale 1999). This species appears to avoid dense layers of
litter while foraging, preferring stands of live vegetation, possibly because dense ground
cover is more difficult to move through (Robbins and Dale 1999).
2.1.3 Clay-colored Sparrow
Clay-colored sparrows are common in low shrub communities of northern
grasslands, but non-prairie habitats include thickets along edges of waterways, secondgrowth areas, and forest edges (Knapton 1994). Territories are used for breeding only and
are mainly influenced by adequate shrub cover for nest sites (Knapton 1994). They are
the only species of common songbirds in this study that nests in shrubs. The preferred
shrub is snowberry (Symphoricarpos occidentalis), most likely because the leaves limit
light penetration more so than other prairie shrubs like silver sagebrush (Artemisia cana)
and greasewood (Sarcobatus vermiculatus) and make it more difficult for predators to
find the nest (Knapton 1994). Establishment and defense of territories is performed by
singing on shrubs near edge of territory (Knapton 1994). Clay-colored sparrows forage
either on or off-territory where the grassland is more open but do not venture far
13
(Knapton 1994). Although strongly dependent on the availability of shrubs for nesting
and territorial displays, this species gleans a wide variety of seeds and insects off grasses
and forbs, rarely foraging within shrubs (Knapton 1994).
2.1.4 Vesper Sparrow
The breeding range of vesper sparrows is wide spread in North America. This
species prefers dry open habitats with patches of short and sparse vegetation, bare
ground, and low to moderate shrub and forb cover (Wiens 1969, Stewart 1975, Jones and
Cornely 2002). Historically, it is presumed that this species may have used open sites
created from disturbances such as fire, bison, or erosion (Jones and Cornely 2002). In
addition, moist areas with dense vegetation are generally avoided (Jones and Cornely
2002). Territories are multi-functional and include grassy areas for nesting, barren areas
for foraging, and taller vegetation or rocks for perching (Jones and Cornely 2002). Nests
are located on the ground in a shallow depression made by the female, usually concealed
by clumps of vegetation (Jones and Cornely 2002). Vegetation used for concealment
includes grasses or forbs, which are also used as the substrate for the nest cup (Jones and
Cornely 2002). Foraging occurs in low ground cover (Wiens 1969, Jones and Cornely
2002), feeding mainly on insects and spiders gleaned off vegetation, but also on a wide
variety of seeds.
2.1.5 Savannah Sparrow
The breeding range of Savannah sparrows is wide spread in North America and
this species is often considered a habitat generalist (Wheelwright and Rising 1993,
14
Johnson 1997, Madden et al. 2000). In grasslands, this species prefers dense ground
vegetation typical of moist meadows or lightly grazed fields, but it also occurs along
roadsides, sedge bogs, edge of salt marshes, and tundra (Wheelwright and Rising 1993).
Available perching sites used for observation or song have reportedly influenced
the location of territories, in addition to topography, which may limit vision
(Wheelwright and Rising 1993). Male Savannah sparrows defend their territory by
moving between song perches that incorporate shrubs, tall forbs and grasses, rocks, or
fence posts (Wiens 1973, Wheelwright and Rising 1993). Females often select nest sites
that are near the edge or outside the male’s territory, which requires males to expand the
boundary and defend new areas (Wheelwright and Rising 1993). Nests are placed on the
ground, usually in a shallow depression made by the female, and either underneath or at
the base of clumps of grass, forbs, or low shrubs (Wheelwright and Rising 1993). Some
nests are open cups, but many nests are hidden well by an additional canopy of dead
vegetation placed on top (Wheelwright and Rising 1993). Important factors in selecting
nest-sites include areas with a dense litter layer, low bare ground component, and cow
dung, where vegetation grows thicker as a result of fertilization (Davis 2005). While
territories are often multi-functional, providing suitable areas for all facets of breeding,
foraging is sometimes done off-territory typically in shorter vegetation, and including
mudflats (Wheelwright and Rising 1993). When foraging, the sparrow walks on the
ground, usually around the perimeter of vegetation clumps (Wiens 1973), and searches
for a variety of seeds, insects, spiders, millipedes, isopods, amphipods, decapods, and
mites (Wheelwright and Rising 1993).
15
2.1.6 Baird’s Sparrow
Breeding habitat of Baird’s sparrow occurs in well-drained mixed-grass and
fescue prairies of the northern Great Plains (Green et al. 2002). Baird’s sparrows have
strong affinities to mid-height grasses (Davis and Duncan 1999, Madden et al. 2000,
Green et al. 2002) with dense ground cover and intermediate litter depths (Johnson 1997,
Davis and Duncan 1999). Lightly grazed or ungrazed prairie in southern Saskatchewan
reportedly provides this type of vegetation structure (Stewart 1975, Johnson 1997, Davis
et al. 1999, Madden et al. 2000).
Singing perches typically consist of shrubs, but singing on the ground or in
clumps of vegetation is also frequent (Green et al. 2002). This sparrow spends most of its
time on the ground, walking and hopping between clumps of vegetation, and usually
remains hidden when not singing on shrubs (Green et al. 2002). Nests are placed on the
ground in a shallow depression that is either natural or made by the female, and within
areas of dense dead vegetation (both litter and standing dead) and fewer amounts of live
vegetation and bare ground (Green et al. 2002). Nests are usually placed beside
overhanging tufts of grass and shrubs or in deep depressions where nest concealment is
provided by the ground (Davis and Sealy 1998, Green et al. 2002). As with nest sites,
foraging sites are also typically within dense vegetation and litter, and avoid unsheltered
areas (Green et al. 2002). This species feeds mainly on insects, but also on a wide variety
of seeds, from the surface of the ground or gleaned from vegetation (Green et al. 2002).
16
2.1.7 McCown’s Longspur
Breeding habitat of McCown’s longspur is restricted to short-grass prairie of the
northern Great Plains, where short vegetation and low ground cover exist (Stewart 1975,
With 1994, Knopf 1996). Territories are established in areas with sparse vegetation such
as hilltops or heavily grazed pastures, and include areas for all necessary facets of
breeding, such as display, nesting, and foraging (With 1994).
Males maintain their territories with an aerial display that is also used in
courtship. During display, the male sings and floats down to the ground with his wings
and tail feathers spread open (With 1994). In addition, the male may sing from perches
such as shrubs or rocks (With 1994). Nests are an open cup placed on the ground in a
shallow depression that is either natural or made by the female. Sometimes nests are
placed in the open, and sometimes they are sheltered by vegetation such as short grasses,
low-lying shrubs, or cactus, and have also been associated with cattle dung (With and
Webb 1993). However, With and Webb (1993) found that often this vegetation near the
nest does not provide shelter from the sun during the majority of the day, suggesting that
exposure of the nest to sunlight might provide a thermal advantage for this species. In
addition, most nests were found on south-facing slopes (With and Webb 1993). Foraging
is done in similar habitat, usually by walking on the ground and searching for seeds or
insects, but sometimes hawking (catching prey in air) or gleaning tactics are used (With
1994).
17
2.1.8 Chestnut-collared Longspur
Breeding habitat of chestnut-collared longspur is restricted to short- and mixedgrass prairies of the northern Great Plains. Preferred habitat includes arid prairies with
vegetation heights no taller than 20-30 cm (Owens and Myres 1973) and minimal litter
accumulation (Hill and Gould 1997, Madden et al. 2000, Fritcher et al. 2004, Davis
2005). Historically, this species probably bred in areas disturbed by bison or fire (Owens
and Myres 1973) and presently breeds in grazed pastures or recently burned grasslands
(Cody 1985, Stewart 1975, Davis et al. 1999, Danley et al. 2004).
Like McCown’s longspurs, male chestnut-collared longspurs defend their
territories with an aerial display that is also used in courtship. The display is similar to
McCown’s longspur, where the male sings while floating to the ground with tail feathers
spread open (Hill and Gould 1997). As with other grassland songbirds, emergent forbs
and shrubs are also used for perching and singing (Wiens 1973). Nests are placed on the
ground in a shallow depression often adjacent to cattle dung, and clumps of grass or
forbs, that sometimes form a canopy over the open cup nest (Hill and Gould 1997). Most
nests are made solely from grasses, sometimes forbs, which are gathered no more than 20
m from the nest site (Hill and Gould 1997). Foraging occurs off-territory on barren
ground, but adjacent to the more grassy breeding areas (Hill and Gould 1997). Chestnutcollared longspur feed on seeds and insects, usually found on the ground surface or
gleaned off vegetation, but sometimes hawk flying insects close to the ground (Hill and
Gould 1997).
18
2.2 Historical Occurrence of Fire and Grazing Disturbances
Disturbances play a main role in the dynamics of grassland ecosystems (Vinton
and Collins 1997). Historically, the Great Plains experienced the most intense and
frequent disturbances of all ecosystems within North America (Knopf and Samson 1997,
Brawn et al. 2001), with the most common disturbances on grassland landscapes
occurring from fire, grazing, and the interactions between these two disturbances
(Higgins 1986, Knopf 1996, Vinton and Collins 1997, Brawn et al. 2001).
The high historical frequency of fire on the Great Plains was attributable to
lightning and Native Americans. Based on the accumulation of litter and vegetation
density, the historic lightning-caused fire return interval for a patch of land in the
northern mixed-grass prairie has been estimated to be five to 10 years (Wright and Bailey
1982, Madden et al. 1999). Fires set by Aboriginal people, however, were much more
frequent than lightning-caused fires, and were intentionally used for modifying grassland
landscapes before European settlement on the northern Great Plains (Higgins 1986,
Bailey 1988). Based on historical accounts by ancestors of Aboriginal people and
European travelers, Higgins (1986) suggests that fire was primarily used to remove fuel
build up and hence prevent wildfires, especially around campsites, and to enhance forage
production for roaming bison and other wildlife. Other uses for burning the northern
prairies included warfare, communicating, warnings, eliminating evidence of campsites
and trails, ceremonies, and pleasure (Higgins 1986). While some fires occurred in the
same location, during the same season, each year, others occurred in differing locations
and frequencies each year, with the majority of these intentional fires being small events
scattered across the landscape (Higgins 1986). Large fires on the grasslands were
19
primarily set by accident or by lightning, and not commonly set intentionally, since they
would have caused widespread starvation among the hunter-gatherer communities
(Higgins 1986). While lightning-caused fires occurred predominately during the summer
months, the small intentional man-set fires occurred in every month, possibly with the
exception of January, and with peak frequencies in spring and fall (Higgins 1986).
Human-set fires were not based on the seasonal patterns of natural lightning fires;
instead, fires were set based on the movement patterns of bison herds to attract the
animals to lush regrowth (Higgins 1986). Thus, the seasonal patterns of fire followed by
grazing dominated the prairie landscape prior to European settlement and the interaction
effects of grazing and burning are evolutionary significant in the northern mixed-grass
prairie.
The interaction of fire and grazing created a mosaic of spatially and temporally
distinct habitat that ranged from heavily disturbed patches to undisturbed patches
(Fuhlendorf et al. 2006). Thus, grassland birds adapted to specific habitat types occurring
along a disturbance continuum (Knopf 1996, Fuhlendorf et al. 2006). In fact, birds that
are endemic to the grasslands are strongly associated with the short and sparse vegetation
structure resulting from the interaction of these disturbances and actually evolved from
the short- and mixed-grass components of the northern prairies (Mengel 1970, Knopf
1996). It has been suggested that the conversion of grasslands towards a more
homogeneous landscape due to fire suppression has contributed to the decline of
grassland bird populations (Brawn et al. 2001, Fuhlendorf et al. 2006) and that periodic
defoliations by disturbances are vital for maintaining biodiversity (Madden et al. 1999). It
is because of this evolution with frequent disturbance that maintaining the natural
20
disturbances on grassland landscapes is essential to the health and integrity of the
ecosystem (Knopf and Samson 1997). Historically, North American grasslands did not
experience prolonged periods of rest, and, therefore, grasslands that are left undisturbed
by fire or grazing and result in later seral vegetation may be unattractive habitat to many
grassland birds (Madden et al. 1999, Askins et al. 2007). Fire is integral to grassland
ecosystems when considering its evolutionary importance (Brawn et al. 2001), the
naturally high frequency (Higgins 1986), and the disturbance-dependent successional
cycle that provides a wide variety of habitat to grassland birds (Brawn et al. 2001). In
fact, some ecologists suggest that in some prairies, fires are ultimately what maintain and
preserve grassland integrity (Higgins 1986, Bragg 1995, Madden et al. 1999, Brennan
and Kuvlesky 2005).
2.3 The Role of Fire in Mixed-Grass Prairies
The direct effects of fire diminish at higher trophic levels. Specifically, fire
directly affects the vegetation community; however, it is not apparent whether there is a
direct effect on the bird community (Vinton and Collins 1997). In general, it has been
suggested that the effects of fire on the bird community are expressed through the
changes in vegetation structure (Vinton and Collins 1997). This observation is consistent
with suggestions that the primary determinant of habitat selection by many grassland
birds is the vegetation structure (Wiens 1969, Bollinger 1995, Madden et al. 2000, Brawn
et al. 2001), because it is what determines available display perches, shelter for nests, and
suitable foraging areas (Wiens 1969, Wiens 1973). Therefore, to understand effects that
21
fire may have on grassland birds, it is essential to understand how fire modifies grassland
vegetation.
In a northern mixed-grass prairie of North Dakota, vegetation became short,
sparse, grass dominated, with low amounts of litter and forbs, six months after a fire
(Madden et al. 1999). Burns older than two years were progressively similar to unburned
prairie and contained vegetation that was taller, denser, shrub and forb dominated, with
higher amounts of litter (Madden et al. 1999). Grassland bird species preferring short and
sparse vegetation are likely to find suitable habitat in recently burned prairie, while
species preferring taller and denser vegetation will inhabit areas of different years postfire. Many of the endemic grassland birds prefer the vegetation characteristic of fire and
grazing defoliations, due to their affinities for shorter and less dense vegetation structure
(Knopf 1996, Madden et al. 2000). For instance, Madden et al. (1999) found that species
richness, and abundances of Sprague’s pipit, Baird’s sparrow, grasshopper sparrow, and
western meadowlark were positively correlated with recent and frequent fire; the
abundance of Savannah sparrow was more associated with intermediate stages of postfire succession, such as six to eight years post-fire; and clay-colored sparrow was
associated with unburned prairie due to preferences for shrubby vegetation.
Grasslands experiencing years of fire suppression tend to be degraded by being
converted to later seral stages, such as invasions of woody plants in temperate regions,
but also taller vegetation, greater canopy cover, and greater litter depth in general (Brawn
et al. 2001, Fritcher et al. 2004, Brennan and Kuvlesky 2005). Thus, on remaining
grassland tracts, it is important to maintain the processes of natural disturbances, such as
fire, because it reintroduces early seral vegetation, which is characteristic of grassland
22
habitat (Lesica and Cooper 1999, Brawn et al. 2001, Fritcher et al. 2004). For instance,
after conducting a series of burns that eliminated the shrub accumulation in a northern
mixed-grass prairie, Madden et al. (1999) found that endemic grassland birds such as
Baird’s sparrow, Sprague’s pipit, and chestnut-collared longspur, increased in numbers.
Therefore, the reintroduction of fire onto lands invaded by woody plants effectively
generates habitat for grassland birds, illustrating that fire can be utilized to revert
grasslands to their characteristic early seral stages.
Fire generates habitat heterogeneity both at the landscape and local scale. At the
landscape level, spatial heterogeneity is created when burning creates new habitat.
Temporal heterogeneity is simultaneously created due to prairie succession within these
burned patches, which further creates new habitat for a variety of species as vegetation
structure changes with time. Brawn et al. (2001) suggest that this successional cycle is
disturbance-dependent and has been shown to be vital in structuring grassland bird
communities.
At the local scale, biodiversity may increase because of a reduction in competitive
dominance of vegetation species (Brawn et al. 2001, Overbeck et al. 2005). Disturbances
such as fire can decrease competitive dominance by altering the levels of resource
availability (Brawn et al. 2001), and increase vegetation diversity by enabling the lowgrowing vegetation to establish after litter removal, and by promoting the establishment
of xeric adapted species (Bailey 1988). In addition, fire sometimes creates patches at the
local level within the burned areas themselves, ranging from sparsely burned to severely
burned vegetation (Lesica and Cooper 1999), which changes the horizontal patchiness of
vegetation and increases litter patchiness (Madden et al. 1999). Thus, burned areas
23
generate habitat heterogeneity because of increased patchiness at both the landscape and
local scales (Lesica and Cooper 1999). This habitat heterogeneity can directly increase
the biodiversity of wildlife, because of increased niche availability (Bollinger 1995, Van
Dyke et al. 2007).
2.4 Combined Role of Fire and Grazing on Mixed-Grass Prairie
Fire typically enhances the species diversity, growth, and reproduction of
vegetation (Collins and Barber 1986), thereby enhancing the quality and productivity of
forage for grazing animals (Wright and Bailey 1982, Biondini et al. 1999). For instance,
as forbs and grasses mature, their structural components become more complex, and their
proportion of digestible nutrients decreases, which causes the herbaceous vegetation to
become fibrous and unpalatable for grazing animals (Bailey 1988). In addition, the
buildup of litter and standing dead vegetation can act as a barrier to grazing (ErichsenArychuk et al. 2002). Burning can increase the quality of the forage by reducing the
vertical and horizontal density of vegetation (Madden et al. 1999), which reduces the
proportion of structural components in the grasses. The quality of the forage after burning
is also improved by increases in the nutrient availability of the soil (Wright and Bailey
1982, Erichsen-Arychuk et al. 2002), which enhances the proportion of digestible
nutrients in the vegetation. Additionally, burning can increase the productivity of the
forage by reducing the amount of litter, thereby increasing the light penetration for
emerging shoots, and warming the soil (Wright and Bailey 1982, Bragg and Steuter
1996). For these reasons, the herbaceous growth after a fire is usually more palatable and
productive. This seems to explain why grazing animals select burned over unburned areas
24
of vegetation, resulting in preferential grazing and increased grazing pressure in the
burned areas (Bailey 1988, Biondini et al. 1999). In addition, grazing on burned prairie
can prolong the improved quality of the forage, which maintains preferential grazing in
those areas (Biondini et al. 1999).
The interaction of fire and grazing generates feedback mechanisms where fire
alters the extent of grazing and grazing alters the extent of fire (Fuhlendorf et al. 2006).
Burned areas that are preferentially grazed maintain low fuel loads and higher amounts of
bare ground. This reduces the probability of a second fire within the patch, whereas the
unburned areas, which are less strongly selected by grazers, continually build up fuel
loads from litter accumulation, which increases the probability of a fire within that area
(Fuhlendorf et al. 2006). When fire and grazing are both present in grassland ecosystems,
these feedback mechanisms generate a mosaic of spatially and temporally distinct patches
of habitat (Fuhlendorf et al. 2006). Thus, the combined effects of grazing and burning
provide a wide range of habitats suitable for a variety of grassland birds.
While burning alone reduces the height and density of vegetation, Danley et al.
(2004) suggest that the added effect of grazing on burned prairie magnifies the extent to
which vegetation height and density are reduced. Vegetation becomes shorter and
sparser, with a higher component of bare ground, in areas where burning and grazing are
both applied, due to preferential grazing by herbivores. As a result, a wider variety of
habitat types are created by the interaction between burning and grazing, rather than
simply applying fire or grazing either on their own or under a uniform pattern
(Fuhlendorf et al. 2006). Creating spatially and temporally distinct patches is valuable for
grassland habitat and grassland bird conservation (Fuhlendorf et al. 2006). Reintroducing
25
fire onto grassland landscapes should therefore be beneficial to grassland birds on both
grazed and ungrazed mixed-grass prairie.
2.5 Effects of Burning and Grazing on Grassland Songbirds
Most birds in the northern mixed-grass prairies respond positively to fire,
although this typically does not occur until one to two years post-burn. This has been
illustrated by Johnson (1997) who studied the long-term effects after burning on
grassland bird densities and discovered that densities of Baird’s sparrow, grasshopper
sparrow, Savannah sparrow, and western meadowlark declined during the first year postfire, peaked between one to five years post-fire, and then declined progressively with the
age of unburned prairie. Similarly, Madden et al. (1999) observed a decrease in
abundance of Baird’s sparrow, grasshopper sparrow, and Sprague’s pipit during the first
year post-fire, but an increase during the second and third years post-fire. Danley et al.
(2004) observed the same pattern regarding abundances of Baird’s sparrow, grasshopper
sparrow, and Sprague’s pipit while examining fire effects on grazed prairie. Baird’s
sparrows have shown preferences for dense ground cover and avoidance of areas with a
high prevalence of bare ground, such as vegetation two to three years post-fire (Johnson
1997). Sprague’s pipits have shown associations with dense ground cover, standing dead
vegetation, and minimal components of bare ground (Robbins and Dale 1999). While
periodic grazing or burning in North Dakota may create these vegetation characteristics,
in drier grassland regions of southern Saskatchewan, lightly grazed or even ungrazed
prairie may provide the most suitable habitat (Stewart 1975, Johnson 1997, Madden et al.
2000). Grasshopper sparrows have been shown to prefer patchy vegetation, with dense
26
ground cover, and moderate litter depths (Johnson 1997). In some instances, grasshopper
sparrows are associated with intermediate vegetation heights (Madden et al. 2000), while
others have shown grasshopper sparrow density to increase with increasing vegetation
height and density (Stewart 1975, Fritcher et al. 2004).
Although Johnson (1997) determined that densities of western meadowlark and
Savannah sparrow were highest between one to five years post-burn, these responses
were relatively small, and the occurrence of these species was still common in unburned
prairie. In other studies, these two songbirds have also shown relatively small responses
(in some cases no response) to successional stages (Fritcher et al. 2004). Western
meadowlarks are typically considered generalists in the grasslands because they find
suitable habitat in several seral stages (Cody 1985, Johnson 1997). Western meadowlarks
have previously shown preferences for prairies with at least moderate litter cover for
constructing nests, which may explain the low density immediately following fire, since
fire removes most litter (Johnson 1997). This may also suggest that two years post-fire in
North Dakota is adequate time for litter build up suitable for nest making. Savannah
sparrows are also common in a variety of seral stages (Madden et al. 2000) and have
shown either no response to burned prairie (Madden et al. 1999) or inconsistent responses
(Cody 1985). However, Madden et al. (1999) have clarified that this may be because
Savannah sparrows prefer intermediate seral stages of post-fire prairie. Brown-headed
cowbirds are also considered habitat generalists since a wide variety of vegetation types
can be used by this species (Madden et al. 1999, Madden et al. 2000). For example,
Johnson (1997) observed that brown-headed cowbird densities were constant in all years
following burning, although decreased slightly during the first year post-burn, which may
27
simply be a reflection of the reduction in potential host nests immediately following fire
(Johnson 1997).
Some birds have shown immediate positive responses to fire. Due to their
associations with short grass, very sparse vegetation, and low litter depths, horned larks
and chestnut-collared longspurs have shown positive responses to early seral stages
(Fritcher et al. 2004), such as immediately after burning (Cody 1985) and preferences for
grazed prairie (Stewart 1975, Davis et al. 1999, Danley et al. 2004). In addition, Danley
et al. (2004) found that horned lark was also most abundant during the first year postburn in burned-grazed prairie compared to burned-only prairie. For this, Danley et al.
(2004) suggest that the added grazing may have reduced the already short and sparse
post-fire vegetation even further, to provide suitable habitat for horned larks due to their
strong association with very short and sparse vegetation. Similarly, McCown’s longspur
also favours habitat in heavily grazed mixed-grass prairie with very short vegetation
(Stewart 1975).
Because most grassland birds are relatively absent from recently burned prairie,
there may be habitat losses immediately following fire. However, this response is usually
short-term (typically only during the first year post-fire) and eventually results in suitable
habitat for the majority of grassland birds (Johnson 1997, Madden et al. 1999, Danley et
al. 2004). However, it is important to note that birds preferring shrubby incursions, like
vesper sparrows and clay-colored sparrows, may suffer more long-term habitat losses.
28
3.0 Methods
3.1 Study Area
3.1.1 Site description
This study was conducted in 2007 and 2008 within the northern mixed-grass
prairie of southwest Saskatchewan, in the East Block of Grasslands National Park of
Canada (GNPC), and neighbouring Mankota Community Pastures (Figure 1). The
landscape of southern Saskatchewan’s mixed-grass prairie consists of rolling topography
with elevations of 800 m to 1000 m above sea level (Parks Canada 2002). Characteristic
vegetation of this area includes upland prairie dominated by grasses such as needle-andthread grass (Stipa comata), blue grama grass (Bouteloua gracilis), western wheatgrass
(Pascopyrum smithii), and June grass (Koeleria macrantha); and lowland shrubs like
silver sagebrush (Artemisia cana), western snowberry (Symphoricarpos occidentalis),
and greasewood (Sarcobatus vermiculatus); Parks Canada 2002).
East Block
GNPC
Mankota
Pastures
Figure 1. Study location in the East Block of Grasslands National Park of Canada and the Mankota
Community Pastures in Southern Saskatchewan (source: Parks Canada 2002).
29
3.1.2 Burning and grazing locations
No grazing has occurred in GNPC since 1985 in most areas, because cattle
grazing was terminated once Parks Canada obtained the land rights from previous
landowners (Parks Canada 2002). The adjacent Mankota pastures, however, contain
season-long cattle grazing at a light to moderate intensity (approximately 25-50%
biomass removal annually). Natural and prescribed burning has occurred rarely around
GNPC for numerous decades due to fire suppression by the surrounding communities (M.
Fitzsimmons, personal communication, November 9, 2007, Ecosystem Scientist with
Parks Canada). Additionally, the suppression of natural fires continues within the GNPC
boundaries (Parks Canada 2002). Nevertheless, in July of 2006 several wildfires occurred
within and around GNPC and the Mankota pastures, which ranged from 12 ha to 750 ha.
These burns generated areas of unburned-grazed (UB-G) and burned-grazed (B-G) prairie
within the Mankota pastures, and burned-ungrazed (B-UG) and unburned-ungrazed (UBUG) within GNPC.
3.1.3 Experimental Design
Wildfires were limited to upland areas; therefore, only upland prairie was
surveyed in the unburned areas. The burns chosen for this study ranged from 70 ha to 750
ha, which ensured that enough area was present in each burn to place three to five
replicate point-count plots for a nested study design, within which songbird surveys and
habitat measurements took place. A total of five burns were used, two in the grazed fields
(Mankota), one in the ungrazed fields (GNPC), and two that occurred half in grazed and
half in ungrazed. The burn that occurred entirely within GNPC was large enough to place
30
two sites approximately 6 km apart (burns 2 and 3, Figure 2), as it was reasonable to
assume the use by birds in these parts of the burn were far enough apart to be considered
independent. Two sites were also placed within each of the two burns that occurred half
in grazed and half in ungrazed fields (burns 1 and 4 in B-UG, 7 and 8 in B-G, Figure 2).
The UB-G and UB-UG sites were set up prior to the wildfires for a larger grazing
experiment being conducted in the same area and consisted of four ~296-ha pastures in
the grazed prairie and nine ~296-ha pastures in the ungrazed prairie. Replicate plots were
placed in all sites, for a total of 16 plots in B-G, 16 in B-UG, 21 in UB-G, and 53 in UBUG (Table 1).
Figure 2. Clustered study design in southern Saskatchewan. The burned-ungrazed plots (burns 1-4) and the
unburned-ungrazed plots (pastures 1-9) are in the East Block of Grasslands National Park of Canada. The
burned-grazed plots (burns 5-8) and the unburned-grazed plots (pastures 10-13) are in the Mankota
Community Pastures.
31
Table 1. Number of sites and plots in each treatment group.
Treatment Group
Site Name
Number of Plots
Burned-Ungrazed
B1
5
B2
5
B3
5
B4
1
Total = 16
Burned-Grazed
B5
5
B6
3
B7
3
B8
5
Total = 16
Unburned-Ungrazed
P1
6
P2
6
P3
6
P4
6
P5
5
P6
6
P7
6
P8
6
P9
6
Total = 53
Unburned-Grazed
P10
6
P11
6
P12
3
P13
6
Total = 21
3.2 Songbird Surveys
To evaluate species diversity and the relative abundances of various grassland
songbirds in response to burning and grazing, I used five-minute, 100-m radius pointcounts in each plot. Although longer count durations increase the probability of detection,
especially for inconspicuous species, it is also subject to increased bias from movement
of birds from outside to inside the plot (Savard and Hooper 1995). Therefore point-count
duration must be short enough to avoid this bias, but long enough to adequately record
the birds present in the plot. Numerous studies have determined that most species and
32
individuals are observed within the first three to five minutes (Ralph et al. 1995), and this
is especially true in open habitats where individuals are more easily detected (Savard and
Hooper 1995). Thus, a five-minute duration was used to ensure detection of most species
and individuals. The use of a 100-m radius has been suggested as the best point-count
size for open habitats such as grasslands. Savard and Hooper (1995) showed that with
most species of grassland birds, a 100-m radius provided a comparable number of
detections as any radius greater than 100 m and is the most effective radius for open
areas.
The centres of the point-count plots were placed at least 300 m apart, so that there
was at least 100 m between the edges of adjacent plots. It was important that conditional
independence between point-counts be maintained so that the same birds were not
counted in adjacent plots or that the singing rates of birds were not affected by the
singing rates in other plots (Pendelton 1995). Reports from previous point-count surveys
suggest that the minimum distance between the centre of adjacent point-counts be 250 m
because most vocalizations do not travel great distances and observers are typically
capable of detecting more than 95% of birds within a 125-m distance (Ralph et al. 1995).
Songbird species observed in each plot were identified by sight or song and the
abundance of each species was recorded. These observations began immediately upon
arrival in the centre of the plot and continued for a five-minute period (Ralph et al. 1995).
Point-count surveys took place during the breeding season (May and June), and were
conducted immediately following sunrise (approximately from 0500 to 1000 CDT),
because this time of day provides the most consistent detection rate (both visibly and
audibly) when surveying breeding birds (Ralph et al. 1995). It is important to note that
33
nesting and territorial behaviour of horned lark can begin as early as March or April
(Stewart 1975, Beason 1995) and so some breeding pairs may have been missed,
however, the survey period did encompass the majority of the breeding time frame
(Stewart 1975, Dickson and Dale 1999). Point-counts were not conducted during rainy,
foggy, or when wind was >20 km/hr, which may impair species identification and
accuracy of distance estimates (Pendelton 1995, Ralph et al. 1995). Observers were
rotated among sites to minimize bias (Pendelton 1995).
Fixed-radius point-count surveys were chosen because they are commonly used in
bird surveys where the main objectives include assessing relative abundance (i.e., number
of birds per plot) or habitat relationships (Pendelton 1995, Toms et al. 2006). Distance
sampling methodology, which is used to estimate absolute densities (i.e., number of birds
per unit area), was not chosen because of the inherent complications associated with
detection probabilities. For instance, detection probabilities assume that 1) birds at zero
distance from the observer are always detected; 2) birds are detected before any
movement response to the presence of the observer; and 3) accurate measurements are
made on the distance to the bird (Rotella et al. 1999). These assumptions, however, are
rarely met in the field. Based on personal observations, visual distance estimations were
affected by hills and valleys, and audible distance estimations were affected by noise
interference from wind, cows, and persistent calling from various loud birds (e.g.,
marbled godwit [Limosa fedoa] and long-billed curlew [Numenius americanus]). In
addition, detection probabilities are based only on the birds that were detected, which can
be considered a biased estimate in itself because it consists of the most detectable
individuals, thereby leading to unreliable detection probabilities of the population and
34
consequently unreliable absolute densities (Efford and Dawson 2009). Because the
objectives of my study were not to assess density, but rather to compare the habitat
associations of grassland birds among different habitat types, absolute density estimates
were not necessary.
3.2.1 Data Organization
In 2007, three rounds of point-count surveys were conducted in the unburned
prairie, between 1) 25 May and 30 May, 2) 1 June and 5 June, and 3) 11 June and 16
June. Only one round was conducted in the burned prairie, however, between 5 June and
8 June, due to time constraints in the 2007 field season. A preliminary review of the 2007
data indicated that using three rounds in the unburned prairie did not provide a similar
number of species compared to using just one round in the unburned prairie; three rounds
provided a greater number of species because more rounds detected the presence of more
birds. As the burned prairie only had one round of point-counts, I chose to use only one
round from the unburned prairie (round #2, surveyed between 1 June and 5 June) for the
analysis of the 2007 data.
In 2008, three rounds of point-count surveys were conducted in the burned prairie
and in pastures 1, 5, 9, and 10-13 of the unburned prairie. In early June 2008, ungrazed
pastures 2, 3, 4, 6, 7, and 8 in GNPC had cattle placed in them for a larger grazing
experiment being conducted and as a result only two rounds of point-counts were
conducted in these pastures. In the burned prairie, surveys were conducted between 1) 27
May 27 and 5 June, 2) 5 June and 9 June, and 3) 9 June and 19 June. In the unburned
prairie, surveys were conducted between 1) 27 May and 29 May, 2) 30 May and 5 June,
35
and 3) 6 June and 10 June. The means from the three rounds at each plot (or two rounds
for plots in pastures 2, 3, 4, 6, 7, and 8) were used in my analyses for 2008.
The diversity measurements used to analyze the songbird community were
species richness, Shannon-Wiener heterogeneity index, and Shannon evenness index.
These are the most commonly used measures to assess species diversity. Richness gives
the absolute number of species found in a specified area and evenness represents the
relative abundance of individuals belonging to each of those species (Peet 1974). The
heterogeneity index (i.e., the Shannon-Wiener index) includes both richness and evenness
in one calculation. It is valuable to assess the heterogeneity and evenness indices in
addition to species richness, since richness alone leaves out pertinent information on the
equitability of a community (Peet 1974). The Shannon indices were chosen for my
analyses because they are sensitive to abundances of rare species (Peet 1974), which is an
important consideration in my study. The equations for the Shannon-Wiener
heterogeneity index (1), and Shannon evenness index (2) are as follows:
H’ = − Σ pi (lnpi)
(1)
J’ = H’/lnS
(2)
For each point-count plot, pi represented the proportion of each species relative to the
total number of individuals, i; and S represented the species count (species richness).
Each diversity measure was calculated for each point-count plot. In the analysis, the
Shannon-Wiener heterogeneity index, H’, was scaled to units of species, by using the
exponential form, eH’ (Peet 1974).
All songbird species observed within the 100-m radius plot were included in the
diversity calculations, except for birds passing by or flying through the plot, because
36
these birds were less likely to be associated with the habitat of the plot as compared with
birds actually within the vegetation (Ralph et al. 1995). However, birds that forage or
display in the air column above the plot (e.g., barn swallows, cliff swallows, chestnutcollared longspurs) were retained in the calculations.
3.3 Habitat Surveys
To evaluate habitat structure, vegetation measurements included vegetation
height-density, maximum vegetation height, and litter depth. Vegetation height-density
was determined from a visual obstruction reading (VOR) on a Robel pole, specifically the
height to where > 50 % of the pole was visually obstructed by vegetation, determined
from a distance of 4 m and a height of 1 m (Robel et al. 1970). A metre stick was used to
measure the litter depth (measured from the surface of the ground to the top of the litter
layer) and maximum vegetation height (determined by measuring the height of the tallest
plant).
In 2007, the vegetation sampling methods were inconsistent between burned and
unburned prairie. Vegetation measurements from the unburned plots were collected from
a separate study (regarding the effects of grazing on the vegetation structure and plant
diversity) that used 10 samples per plot, all placed within a 20 x 50 m rectangle in the
southern portion of each plot (Figure 3A). In the burned prairie, time restraints in the
2007 field season limited data collection to only four samples per plot, one in each
cardinal direction at random distances from the centre of the plot (Figure 3B). In 2008,
sampling locations were adjusted to ensure all plots had the same number of replicate
samples and that sample locations were consistent from plot to plot. The vegetation in
37
2008 was measured from eight samples in each plot (both burned and unburned), placed
at 50 m and 100 m from the centre of the plot, in the four cardinal directions (Figure 3C).
Additional percent ground cover measurements were collected in 2008 using 1.0
m x 0.5 m frames for structural characteristics including litter, standing dead vegetation,
exposed bare ground, club moss/lichen, rock, animal waste, grass, forbs, cactus, and
shrubs. Cover categories were visually estimated using the following modified
Daubenmire scale: 1, >0-0.1%; 2, 0.1–1%; 3, 1-3%; 4, 3-10%; 5, 10-25%; 6, 25-50%; 7,
50-75%; 8, 75-95%; and 9, 95-100% (Henderson 2006).
To determine ground cover estimates, I viewed the ground cover from above the
vegetation and recorded only what was exposed, and did not rake the litter layer aside to
take into account the unexposed vegetation or bare ground hidden below. For the
purposes of my study, I was interested only in the structural components of ground cover,
such that finding small plants or bare ground concealed by litter was irrelevant. From the
perspective of a small ground-dwelling bird using grassland vegetation, the amount of
exposed bare ground reveals more about the structure of the habitat than the amount of
bare ground hidden underneath a dense layer of litter.
In 2007, vegetation measurements in unburned prairie were taken between 13
June and 18 July, and in burned prairie between 27 August and 28 August. In 2008, all
measurements were taken twice, once between 20 May and 24 May, and again between
28 July and 29 July. For most vegetation surveys, measurements are typically conducted
at the end of summer, and not usually during the same time period as point-count
surveys, which is largely because of time constraints or a limited number of surveyors
(e.g., Madden et al. 2000, Grant et al. 2004). However, because time and assistance were
38
available in 2008, vegetation measurements were collected at the end of May, during the
same time period as the point-count surveys. Vegetation was measured again at the end
of July for comparisons between the May and July measurements to address whether
patterns in the songbird community were more closely related to the habitat structure
present during breeding habitat selection, rather than later in the summer. Habitat samples
from 2007 and May 2008 were located in all plots. In July 2008, ungrazed pastures 2, 3,
4, 6, 7, and 8 in GNPC had cattle placed in them for a larger grazing experiment that was
being conducted and were therefore not used in the July habitat surveys.
Figure 3. Diagram of vegetation subplots for the habitat surveys from A ten samples in the unburned plots
in 2007, all placed within a 20 x 50 m rectangle in the southern portion of each plot, B four samples in the
burned plots in 2007, one in each cardinal direction at random distances from the centre of the plot, and C
eight samples in all plots in 2008, placed at 50 m and 100 m from the centre of the plot, in the four cardinal
directions. x = plot centre, squares = frame locations.
3.3.1 Data Organization
For my analyses, the means of each vegetation variable from the vegetation
subplots were used to examine the vegetation structure per plot. With regards to the
ground cover variables, the modified Daubenmire scale values were converted to the
midpoint value of the cover range. Because the sample size and survey methods for the
39
vegetation measurements were different in each year, I chose to analyze the effects of
burning and grazing on the vegetation in 2007 and 2008 separately, and qualitatively
compare the vegetation between years.
3.4 Data Analysis
3.4.1 Songbird Community and Habitat Structure
For the statistical analysis of the songbird community, I chose to use a generalized
linear mixed model approach. The mixed models method was beneficial for my clustered
study design because it incorporated a random, or grouping, variable to control for
autocorrelation among plots within sites. In my study, the grouping variable was the
pasture or burn site, which contained the replicate plots. The use of mixed models in
ecological studies with clustered study designs is becoming more popular compared to
other statistical methods like ANOVA. Using ANOVA for a clustered study design,
where the plot values are averaged within each site, loses information from the variation
that exists between plots, and the statistical power is reduced by consequently examining
a smaller sample size. In a mixed models approach, however, all the information gathered
from the plot scale can be retained and the statistical power to determine significance can
be maximized with a larger sample size (Quinn and Keough 2002).
Three models were used: 1) Treatment Model, 2) Vegetation Structure Model,
and 3) Structural Cover Model (Table 2). The songbird community in 2007 and 2008
was analyzed with the three models by examining species richness, Shannon-Wiener
heterogeneity index, Shannon evenness index, and the relative abundances of the most
40
common songbirds observed in the study area: horned lark, Sprague’s pipit, Savannah
sparrow, Baird’s sparrow, and chestnut-collared longspur. The 2008 analyses also
included the occurrence of clay-colored sparrow, vesper sparrow, and McCown’s
longspur. In 2007, clay-colored sparrow, vesper sparrow, and McCown’s longspur had
low abundances in the study area (these species were only present in one or two plots in
each treatment), such that analysis of these species was either not meaningful or not
possible. In 2008, the abundances were still low, but high enough to be converted to
presence/absence data and analyzed with a binomial distribution. Other songbird species
observed during point-counts (e.g., grasshopper sparrow, western meadowlark, brownheaded cowbird) had much lower abundances, such that the species was either absent
from one or more treatment groups or only present in one or two plots of a treatment
group. These species were included in the diversity calculations (see Appendices I & II
for a list of species included in analyses), but were not analyzed individually because
their relative abundances were too low to be meaningful. In addition, the Treatment
Model was used to analyze the effect of burning and grazing treatments on each habitat
structure variable used to describe the songbird community.
Table 2. Predictor variables in each Model used to assess the grassland songbird community.
Treatment Model
Vegetation Structure Model
a
Structural Cover Model
b
Burning Main Effect
Maximum Vegetation Height
% Litter Cover
Grazing Main Effect
Visual Obstruction Reading
% Standing Dead Cover
Burning-Grazing Interaction
Litter Depth
% Exposed Bare Ground
% Grass Cover
% Forb Cover
% Shrub Cover
a
b
Data collected in 2008 only.
Percentage of ground cover within 1.0 m x 0.5 m frame.
41
I used treatment contrasts to examine differences in songbird diversity among
treatment groups. These additional models looked at species richness and the ShannonWiener heterogeneity index in UB-UG compared to B-G, B-UG, and UB-G, and in B-UG
compared to B-G, UB-G, and UB-UG. Significant effects were determined using an αvalue of 0.10. All statistical analyses were carried out using S-PLUS 8.0.
3.4.2 Habitat Associations for May versus July 2008
Additional Vegetation Structure and Structural Cover Models were conducted for
species richness, heterogeneity, evenness, and the relative abundances and occurrences of
individual species using the vegetation measurements collected in July 2008. The purpose
of these tests was to evaluate whether May or July vegetation structure correlated more
closely with the structure of the songbird community.
3.4.3 Year Effect
I analyzed the songbird community of each year separately, rather than combining
the 2007 and 2008 data and included year as a predictor variable in each of the models.
To combine the data from both years into a single test, the collection methods and the
sample sizes of each year must be same; otherwise, there can be confounding effects.
Because collection methods differed among years for both the point-count surveys and
habitat surveys, conducting the analyses for 2007 and 2008 separately was necessary.
However, the effect of year is still an important consideration when ecological
studies are conducted over two or more years. Thus, a Treatment-Year Interaction Model
(burning x grazing x year) was conducted for species richness and heterogeneity to
42
examine any effects of year on the songbird community. Because the number of pointcount rounds used in each year was different, I used only one round from the 2008 data
(round #2, surveyed between 30 May and 5 June in unburned prairie and between 5 June
and 9 June in burned prairie) for comparability with the 2007 data in this analysis. Year
did not have a significant influence on the effect of burning and grazing on the songbird
community (species richness; linear mixed model, β = 0.3750, SE = 0.3851, df = 187, P =
0.3314, species heterogeneity; linear mixed model, β = 0.3606, SE = 0.3480, df = 187, P
= 0.3015) and accordingly was not included in subsequent analyses. I chose to retain the
three rounds used in 2008 for my analyses (i.e., rather than only using one round for
comparability with 2007) because more rounds can detect the presence of more species
and thus give a better representation of the population being sampled.
43
4.0 Results
4.1 Songbird Community Relationship with Burning and Grazing
4.1.1 First Year Post Burn (2007)
In 2007, species richness and heterogeneity were significantly explained by the
burning-grazing interaction (Table 3), such that burning increased species diversity in
ungrazed prairie but did not affect diversity levels on grazed prairie (Figure 4). Prairie in
UB-UG had the lowest species richness and heterogeneity of the four treatment groups
(Appendix III). Prairie in B-UG, B-G, and UB-G had similarly high species richness and
heterogeneity levels (Figure 4), although only B-UG and UB-G had species richness and
heterogeneity that was significantly greater than UB-UG levels (Table 4, Figure 4).
Evenness was similar among the four treatment groups (Table 3, Figure 4, Appendix III).
The relative abundances of Sprague’s pipit and Baird’s sparrow were significantly
lower on burned prairie (Table 3, Figures 5, 6). For both species, the effect size of
burning appeared to be equal in both grazed and ungrazed prairie (Figure 5). In addition,
the relative abundances of Sprague’s pipit and Baird’s sparrow were not significantly
different between UB-G and UB-UG (Figure 5). Chestnut-collared longspur relative
abundance was significantly greater in burned prairie (Table 3, Figures 5, 6), with an
approximately equal effect size of burning in both grazed and ungrazed prairie (Figure 5).
This species was also more abundant in UB-G compared to UB-UG (Figure 5). The
relative abundance of horned larks was significantly influenced by the interaction of
burning and grazing (Table 3). The B-UG, UB-G, and B-G prairies had high abundances
of horned larks, while UB-UG prairie had much lower abundances (Figures 5, 6). The
44
relative abundance of Savannah sparrows was similar among the four treatment groups
(Table 3, Figure 6).
Table 3. Significant treatment effects (burning main effect, grazing main effect, or burning-grazing
interaction) on the songbird community for upland prairie in the Mankota community pastures and the East
Block of Grasslands National Park of Canada, June 2007.*
Songbird Community
Variable
Estimate (β)
Standard Error P Value
Richness
Burning-Grazing Interaction
-1.1096
0.5548
0.0617
Heterogeneity
Burning-Grazing Interaction
-1.0550
0.4678
0.0376
Horned lark
Burning-Grazing Interaction
-1.6698
0.7067
0.0304
Sprague’s pipit
Burning
-1.1255
0.2916
0.0013
Baird’s sparrow
Burning
-0.7997
0.3415
0.0316
Chestnut-collared longspur
Burning
1.5611
0.3115
0.0017
* Songbird community parameters that were not significantly affected by treatment included
evenness and the relative abundance of Savannah sparrow.
Table 4. Comparison of songbird diversity levels for 1) burned-ungrazed prairie vs. burned-grazed,
unburned-grazed, and unburned-ungrazed, and 2) unburned-ungrazed prairie vs. burned-grazed, burnedungrazed, and unburned-grazed, for upland prairie in the Mankota community pastures and the East Block
of Grasslands National Park of Canada, June 2007.
a
Treatment groups significantly different from burned-ungrazed prairie
Songbird Community
Treatment Group
Estimate (β)
Standard Error
P Value
Richness
Unburned-ungrazed
-0.9479
0.3633
0.0183
Heterogeneity
Unburned-ungrazed
-0.8455
0.3061
0.0133
b
Treatment groups significantly different from unburned-ungrazed prairie
Songbird Community
Treatment Group
Richness
Burned-ungrazed
0.9479
0.3633
0.0183
Unburned-grazed
0.6616
0.3370
0.0662
Burned-ungrazed
0.8455
0.3061
0.0133
Unburned-grazed
0.5590
0.2824
0.0642
Heterogeneity
a
b
Estimate (β)
Standard Error
P Value
Burned-grazed and unburned-grazed were not significantly different from burned-ungrazed
Burned-grazed was not significantly different from unburned-ungrazed
45
Richness
Heterogeneity
4
Average number of species per plot
Average number of species per plot
5
A
A
AB
B
3
2
1
0
Grazed
5
4
3
A
A
AB
B
2
1
0
Ungrazed
Grazed
Ungrazed
Evenness
Average species evenness per plot
1.00
0.95
Burned
0.90
Unburned
0.85
0.80
Grazed
Ungrazed
Figure 4. Average songbird species per plot (3.2 ha) for species richness, Shannon-Wiener heterogeneity
index (scaled to units of species), and evenness of species distributions, in burned-grazed, burned-ungrazed,
unburned-grazed, and unburned-ungrazed upland prairie in the Mankota Community Pastures and the East
Block of Grasslands National Park of Canada, June 2007. Error bars indicate 90% confidence intervals of
the means. Letters indicate significant differences (P < 0.10).
46
Average number of individuals per plot (3.2 ha)
2.0
Horned Lark
3.0
Sprague's Pipit
2.0
1.0
1.0
0.0
0.0
Grazed
Ungrazed
Baird's Sparrow
3.0
Grazed
4.0
Ungrazed
Chestnut-collared Longspur
3.0
2.0
Burned
2.0
Unburned
1.0
1.0
0.0
0.0
Grazed
Ungrazed
Grazed
Ungrazed
Figure 5. Average number of individuals per plot (3.2 ha) for horned lark, Sprague’s pipit, Baird’s
sparrow, and chestnut-collared longspur, in burned-grazed, burned-ungrazed, unburned-grazed, and
unburned-ungrazed upland prairie in the Mankota Community Pastures and the East Block of Grasslands
National Park of Canada, June 2007. Error bars indicate 90% confidence intervals of the means.
3 .5
Average number of individuals per plot
3 .0
H o rn e d
la r k
2 .5
S p ra g u e 's
p ip it
2 .0
S a va n n a h
s p a r ro w
B a ir d 's
s p a r ro w
1 .5
C h e s tn u tc o lla r e d
lo n g s p u r
1 .0
0 .5
0 .0
B u r n e d -G r a z e d
B u r n e d -U n g r a z e d
U n b u r n e d -G r a z e d
U n b u r n e d -U n g r a z e d
Figure 6. Average number of individuals per plot (3.2 ha) for the five most common species, horned lark,
Sprague’s pipit, Savannah sparrow, Baird’s sparrow, and chestnut-collared longspur, in burned-grazed,
burned-ungrazed, unburned-grazed, and unburned-ungrazed upland prairie in the Mankota Community
Pastures and the East Block of Grasslands National Park of Canada, June 2007.
47
4.1.2 Second Year Post-Burn (2008)
Consistent with the 2007 observations, burning influenced species richness and
heterogeneity differently in grazed compared with ungrazed pastures (Table 5, Figure 7).
Once again, prairie in UB-UG had the lowest species richness and heterogeneity of the
four treatment groups (Appendix IV). Prairie in B-UG and B-G had similarly high
species richness, although only species richness in B-UG was significantly greater than
UB-UG (Table 6, Figure 7). Species richness in UB-G was not significantly different
than species richness in UB-UG (Table 6). Species heterogeneity in B-UG was
significantly higher than in the other treatment groups, which were not significantly
different from each other (Table 6, Figure 7). Unlike 2007, evenness in 2008 showed a
significant interaction between burning and grazing, where evenness was highest in
burned-ungrazed prairie and lowest in burned-grazed prairie (Figure 7, Appendix IV).
Sprague’s pipit and Baird’s sparrow relative abundances were still significantly
lower in burned prairie (Table 5, Figures 8, 9), and the effect size of this decrease
remained similar in both the grazed and ungrazed prairie (Figure 8). Chestnut-collared
longspurs were significantly affected by the burning-grazing interaction (Table 5, Figure
8), such that their relative abundances were significantly lower in UB-UG, while higher
and more similar among the other three treatment groups (Figures 8, 9). The occurrence
of vesper sparrows was significantly higher in burned prairie (Table 5). The occurrences
of clay-colored sparrows and McCown’s longspurs, and the relative abundances of
horned larks and Savannah sparrows, were similar among the four treatment groups in
2008 (Table 5, Appendix IV). Horned larks, however, maintained a pattern in 2008
48
similar to that observed in 2007, where burned or grazed prairie had greater relative
abundances of horned larks compared to UB-UG (Figures 8, 9).
Table 5. Significant treatment effects (burning main effect, grazing main effect, or burning-grazing
interaction) on the songbird community for upland prairie in the Mankota community pastures and the East
Block of Grasslands National Park of Canada, May – June 2008.*
Songbird Community
Variable
Richness
Burning-Grazing Interaction
-1.4990
0.7534
0.0630
Heterogeneity
Grazing
-1.0919
0.4559
0.0284
Burning-Grazing Interaction
-1.6302
0.5757
0.0115
Grazing
-0.0551
0.0309
0.0924
Burning-Grazing Interaction
-0.0701
0.0393
0.0922
Sprague’s pipit
Burning
-0.8268
0.2906
0.0112
Vesper sparrow
Burning
1.9866
0.9795
0.0585
Baird’s sparrow
Burning
-0.6037
0.3233
0.0792
Chestnut-collared longspur
Burning-Grazing Interaction
-0.9864
0.3348
0.0090
Evenness
Estimate (β)
Standard Error P Value
* Songbird community parameters that were not significantly affected by treatment included
the relative abundances of horned lark, clay-colored sparrow, Savannah sparrow, and
McCown’s longspur.
Table 6. Comparison of songbird diversity levels for 1) burned-ungrazed prairie vs. burned-grazed,
unburned-grazed, and unburned-ungrazed, and 2) unburned-ungrazed prairie vs. burned-grazed, burnedungrazed, and unburned-grazed, for upland prairie in the Mankota community pastures and the East Block
of Grasslands National Park of Canada, May – June 2008.
a
Treatment groups significantly different from burned-ungrazed prairie
Songbird Community
Treatment Group
Richness
Unburned-grazed
-1.0430
0.5762
0.0880
Unburned-ungrazed
-1.6258
0.4938
0.0043
Burned-grazed
-1.0919
0.4559
0.0284
Unburned-grazed
-0.9978
0.4403
0.0368
Unburned-ungrazed
-1.5361
0.3773
0.0008
Heterogeneity
Estimate (β)
Standard Error
P Value
b
Treatment groups significantly different from unburned-ungrazed prairie
Songbird Community
Treatment Group
Richness
Burned-ungrazed
1.6258
0.4939
0.0043
Heterogeneity
Burned-ungrazed
1.5361
0.3773
0.0008
a
b
Estimate (β)
Standard Error
P Value
Burned-grazed was not significantly different from burned-ungrazed for species richness
Burned-grazed and unburned-grazed were not significantly different from unburned-ungrazed
49
Richness
A
6
AB
5
B
B
4
3
2
1
0
Grazed
Heterogeneity
7
Average number of species per plot
Average number of species per plot
7
6
A
5
B
4
B
B
3
2
1
0
Ungrazed
Grazed
Ungrazed
Evenness
Average species evenness per plot
1.00
0.95
Burned
0.90
Unburned
0.85
0.80
Grazed
Ungrazed
Figure 7. Average songbird species per plot (3.2 ha) for species richness, Shannon-Wiener heterogeneity
index (scaled to units of species), and evenness of species distributions, in burned-grazed, burned-ungrazed,
unburned-grazed, and unburned-ungrazed upland prairie in the Mankota Community Pastures and the East
Block of Grasslands National Park of Canada, May – June 2008. Error bars indicate 90% confidence
intervals of the means. Letters indicate significant differences (P < 0.10).
50
Average number of individuals per plot (3.2 ha)
Horned Lark
Sprague's Pipit
2.0
2.0
1.0
1.0
0.0
0.0
Grazed
Ungrazed
Grazed
Baird's Sparrow
Ungrazed
Chestnut-collared Longspur
4.0
3.0
3.0
2.0
Burned
2.0
Unburned
1.0
1.0
0.0
0.0
Grazed
Ungrazed
Grazed
Ungrazed
Figure 8. Average number of individuals per plot (3.2 ha) for horned lark, Sprague’s pipit, Baird’s
sparrow, and chestnut-collared longspur, in burned-grazed, burned-ungrazed, unburned-grazed, and
unburned-ungrazed upland prairie in the Mankota Community Pastures and the East Block of Grasslands
National Park of Canada, May – June 2008. Error bars indicate 90% confidence intervals of the means.
3 .0
Average number of individuals per plot
2 .5
H o rn e d
la rk
2 .0
S p ra g u e 's
p ip it
S a va n n a h
s p a rro w
1 .5
B a ird 's
s p a rro w
C h e s tn u tc o lla re d
lo n g s p u r
1 .0
0 .5
0 .0
B u r n e d -G r a z e d
B u rn e d -U n g r a z e d
U n b u r n e d -G ra z e d
U n b u rn e d -U n g r a z e d
Figure 9. Average number of individuals per plot (3.2 ha) for the five most common species, horned lark,
Sprague’s pipit, Savannah sparrow, Baird’s sparrow, and chestnut-collared longspur, in burned-grazed,
burned-ungrazed, unburned-grazed, and unburned-ungrazed upland prairie in the Mankota Community
Pastures and the East Block of Grasslands National Park of Canada, May – June 2008.
51
4.2 Songbird Community Relationship with Habitat Structure
4.2.1 First Year Post-Burn (2007)
Maximum vegetation height, VOR, and litter depth measurements did not
significantly explain species richness, heterogeneity, or evenness in 2007 (Table 7).
Conversely, these vegetation measurements significantly influenced the relative
abundances of all species except Savannah sparrow (Table 7).
Table 7. Significant songbird community relationships with vegetation structure for upland prairie in the
Mankota community pastures and the East Block of Grasslands National Park of Canada, June – July
2007.*
Songbird Community
Variable
Estimate (β)
Horned lark
Visual Obstruction Reading
Standard Error P Value
0.7749
0.3688
0.0387
Litter Depth
-0.2048
0.0818
0.0143
Sprague’s pipit
Litter Depth
0.1777
0.0765
0.0227
Baird’s sparrow
Litter Depth
0.2808
0.1157
0.0174
Chestnut-collared longspur
Maximum Vegetation Height
0.0437
0.0260
0.0963
Litter Depth
-0.3542
0.1465
0.0178
* Songbird community parameters that were not significantly related to 2007 vegetation
measurements included species richness, heterogeneity, evenness, and the relative abundance
of Savannah sparrow.
4.2.2 Second Year Post-Burn (2008)
Similar to the 2007 observations, species richness and evenness were not related
to maximum vegetation height, VOR, or litter depth (Table 8). Species heterogeneity,
however, significantly decreased as litter depth increased (Table 8). Vegetation structure
in 2008 continued to significantly influence the relative abundances of individual species,
including Savannah sparrow (Table 8). Clay-colored sparrows, however, did not show a
significant relationship with any of the three structural measurements and the Vegetation
Structure Model did not converge for vesper sparrow and McCown’s longspur.
52
Species richness, heterogeneity, and evenness appear to be more correlated with
the structural characteristics of ground cover rather than the height and density of
vegetation (Table 9). Likewise, the relative abundances and occurrences of all species
analyzed in the Structural Cover Model were significantly related to at least one ground
cover variable (Table 9).
Table 8. Significant songbird community relationships with vegetation structure for upland prairie in the
Mankota community pastures and the East Block of Grasslands National Park of Canada, May – June
2008.*
Songbird Community
Variable
Estimate (β)
Standard Error P Value
Heterogeneity
Litter Depth
-0.1087
0.0646
0.0961
Horned lark
Maximum Vegetation Height
-0.0242
0.0116
0.0393
Visual Obstruction Reading
-0.4700
0.1348
0.0008
Sprague’s pipit
Litter Depth
0.0814
0.0380
0.0354
Savannah sparrow
Maximum Vegetation Height
0.0279
0.0106
0.0100
Litter Depth
-0.0527
0.0268
0.0522
Baird’s sparrow
Maximum Vegetation Height
0.0357
0.0187
0.0599
Chestnut-collared longspur
Visual Obstruction Reading
-0.7709
0.2603
0.0040
Litter Depth
-0.1124
0.0572
0.0526
* Songbird community parameters that were not significantly related to 2008 vegetation
structure measurements included species richness, evenness, and the occurrence of claycolored sparrow. The Model did not converge for vesper sparrow and McCown’s longspur.
Table 9. Significant songbird community relationships with ground cover for upland prairie in the Mankota
community pastures and the East Block of Grasslands National Park of Canada, May – June 2008.
Songbird Community
Variable
Richness
% Litter Cover
-0.0185
0.0085
0.0321
% Exposed Bare Ground
0.0241
0.0130
0.0675
% Grass Cover
-0.0329
0.0179
0.0692
% Forb Cover
0.0738
0.0355
0.0411
% Shrub Cover
0.1474
0.0617
0.0193
% Exposed Bare Ground
0.0305
0.0103
0.0041
% Shrub Cover
0.1190
0.0488
0.0170
% Litter Cover
0.0009
0.0004
0.0326
% Grass Cover
0.0026
0.0009
0.0034
% Exposed Bare Ground
0.0125
0.0053
0.0214
% Forb Cover
0.0344
0.0146
0.0208
Heterogeneity
Evenness
Horned lark
Estimate (β)
53
Standard Error P Value
Table 9 Continued.
Songbird Community
Variable
Sprague’s pipit
% Litter Cover
0.0092
0.0039
0.0205
% Standing Dead Cover
0.0173
0.0091
0.0601
% Exposed Bare Ground
-0.0124
0.0060
0.0415
Clay-colored sparrow
% Exposed Bare Ground
0.1077
0.0329
0.0016
Vesper sparrow
% Litter Cover
-0.0677
0.0207
0.0016
% Exposed Bare Ground
0.0980
0.0357
0.0075
% Grass Cover
-0.1186
0.0409
0.0048
% Forb Cover
0.2419
0.0902
0.0089
% Shrub Cover
0.3522
0.1513
0.0224
Savannah sparrow
% Grass Cover
0.0193
0.0065
0.0037
Baird’s sparrow
% Litter Cover
0.0133
0.0051
0.0108
% Exposed Bare Ground
-0.0231
0.0075
0.0030
% Litter Cover
-0.0589
0.0186
0.0021
% Grass Cover
-0.0866
0.0404
0.0352
% Litter Cover
-0.0247
0.0060
0.0001
% Exposed Bare Ground
0.0160
0.0087
0.0683
% Grass Cover
-0.0226
0.0130
0.0864
% Forb Cover
0.0548
0.0231
0.0201
McCown’s longspur
Chestnut-collared longspur
Estimate (β)
Standard Error P Value
4.3 Habitat Structure Relationship with Burning and Grazing
4.3.1 First Year Post-burn (2007)
Vegetation structure was significantly altered in prairies burned the previous year
(Table 10, Appendix V). The effect size of burning was similar on both grazed and
ungrazed land (Figure 10).
Table 10. Significant treatment effects (burning main effect, grazing main effect, or burning-grazing
interaction) on the vegetation structure for upland prairie in the Mankota community pastures and the East
Block of Grasslands National Park of Canada, July 2007.
Vegetation Measurement
Variable
Estimate (β)
Standard Error P Value
Maximum Vegetation Height Burning
-0.1037
0.0569
0.0861
Visual Obstruction Reading
Burning
-0.9323
0.1780
0.0001
Litter Depth
Burning
-2.0319
0.2686
<0.0001
54
Maximum Vegetation Height
Visual Obstruction Reading
Height to complete obstruction (cm)
Height of tallest plant (cm)
60
50
40
30
20
10
0
Grazed
14
12
10
8
6
4
2
0
Ungrazed
Grazed
Ungrazed
Litter Depth
10
9
Litter depth (cm)
8
7
6
Burned
5
Unburned
4
3
2
1
0
Grazed
Ungrazed
Figure 10. Average vegetation structure measurements per plot (3.2 ha) for maximum vegetation height
(cm), visual obstruction reading (cm), and litter depth (cm), in burned-grazed, burned-ungrazed, unburnedgrazed, and unburned-ungrazed upland prairie in the Mankota Community Pastures and the East Block of
Grasslands National Park of Canada, July 2007. Error bars indicate 90% confidence intervals of the means.
55
4.3.2 Second Year Post-Burn (2008)
In 2008, both burning and the burning-grazing interaction significantly influenced
vegetation structure (Figure 11) and ground cover (Figure 12), although maximum
vegetation height and standing dead cover were not influenced by these factors (Table
11). Ground cover in burned plots on average had a similar structural composition
between B-G and B-UG, as did unburned plots between UB-G and UB-UG (Figure 13),
suggesting that grazing had little effect on ground cover. The predominant changes in
cover from burning appeared to be increased exposed bare ground, increased grass cover,
and decreased litter cover (Figure 13, Appendix VI). On average, 50% ground cover in
the burned prairie was composed of grass and exposed bare ground. In contrast, around
50% ground cover in unburned prairie consisted of litter.
Table 11. Significant treatment effects (burning main effect, grazing main effect, or burning-grazing
interaction) on the vegetation structure and ground cover for upland prairie in the Mankota community
pastures and the East Block of Grasslands National Park of Canada, May 2008.*
Vegetation Measurement
Variable
Estimate (β)
Standard Error P Value
Visual Obstruction Reading
Burning-Grazing Interaction
0.5854
0.3339
0.0976
Litter Depth
Burning-Grazing Interaction
1.1541
0.4663
0.0241
% Litter Cover
Burning
-1.1551
0.1776
<0.0001
Burning-Grazing Interaction
0.6066
0.2542
0.0290
% Exposed Bare Ground
Burning
1.7589
0.4633
0.0014
% Grass Cover
Burning
0.5906
0.2081
0.0114
% Forb Cover
Burning-Grazing Interaction
-0.5286
0.2727
0.0693
% Shrub Cover
Burning-Grazing Interaction
1.7218
0.9691
0.0935
* Vegetation measurements that were not significantly affected by treatment included the
maximum vegetation height and the percentage of standing dead cover.
56
Maximum Vegetation Height
Visual Obstruction Reading
9
Height to complete obstruction (cm)
Height of tallest plant (cm)
50
40
30
20
10
0
Grazed
8
7
6
5
4
3
2
1
0
Ungrazed
Grazed
Ungrazed
Litter Depth
7
Litter depth (cm)
6
5
4
Burned
Unburned
3
2
1
0
Grazed
Ungrazed
Figure 11. Average vegetation structure measurements per plot (3.2 ha) for maximum vegetation height
(cm), visual obstruction reading (cm), and litter depth (cm), in burned-grazed, burned-ungrazed, unburnedgrazed, and unburned-ungrazed upland prairie in the Mankota Community Pastures and the East Block of
Grasslands National Park of Canada, May 2008. Error bars indicate 90% confidence intervals of the means.
57
Litter Cover
Standing Dead Cover
18
70
16
60
14
50
12
40
10
30
8
6
20
4
10
2
0
0
Grazed
Ungrazed
Grazed
Exposed Bare Ground
Ground cover (%)
30
Ungrazed
Grass Cover
40
35
25
30
20
25
15
20
15
10
10
5
5
0
0
Grazed
Ungrazed
Grazed
Forb Cover
Ungrazed
Shrub Cover
3
10
9
8
7
2
6
Burned
5
Unburned
4
1
3
2
1
0
0
Grazed
Ungrazed
Grazed
Ungrazed
Figure 12. Average percent ground cover per plot (3.2 ha) for litter cover, standing dead vegetation cover,
exposed bare ground, grass cover, forb cover, and shrub cover in burned-grazed, burned-ungrazed,
unburned-grazed, and unburned-ungrazed upland prairie in the Mankota Community Pastures and the East
Block of Grasslands National Park of Canada, May 2008. Error bars indicate 90% confidence intervals of
the means.
58
70
60
E x p . B are
G ro u n d
50
Ground cover (%)
G rass
F o rb s
40
S h ru b s
30
L itter
S tan d in g
D e ad
20
10
0
B u rn ed -G razed
B u rn ed -U n g raze d
U n b u rn e d -G raz ed
U n b u rn ed -U n g razed
Figure 13. Comparison of average percent ground cover per plot (3.2 ha) for burned-grazed, burnedungrazed, unburned-grazed, and unburned-ungrazed upland prairie in the Mankota Community Pastures
and the East Block of Grasslands National Park of Canada, May 2008.
4.4 Songbird Community Relationship with July 2008 Habitat Structure
In general, vegetation structure and ground cover measured in July showed fewer
significant relationships with the songbird community (Tables 12, 13), compared with the
vegetation structure and ground cover that was measured in May (Tables 8, 9) although
there were some exceptions. For instance, in May, no significant vegetation structure
relationship was observed for evenness and clay-colored sparrow occurrence, yet in July,
evenness was related to maximum vegetation height, and clay-colored sparrow
occurrence was related to maximum vegetation height and VOR. With regards to ground
cover in May, the relative abundances of Savannah sparrow was related only to grass
cover, however, in July, this species was related to two cover variables; forb litter cover.
The Vegetation Structure Model did not converge for McCown’s longspur and
both models did not converge for vesper sparrow.
59
Table 12. Significant songbird community relationships with July vegetation structure for upland prairie in
the Mankota community pastures and the East Block of Grasslands National Park of Canada, May – July
2008.*
Songbird Community
Variable
Estimate (β)
Standard Error P Value
Evenness
Maximum Vegetation Height
0.0035
0.0018
0.0562
Horned lark
Visual Obstruction Reading
-0.3958
0.1948
0.0473
Clay-colored sparrow
Maximum Vegetation Height
-0.1439
0.0789
0.0739
Visual Obstruction Reading
2.1338
1.1268
0.0638
Savannah sparrow
Maximum Vegetation Height
0.0279
0.0106
0.0100
Baird’s sparrow
Maximum Vegetation Height
0.0388
0.0231
0.0998
* Songbird community parameters that were not significantly related to July 2008 vegetation
measurements included species richness, heterogeneity, and the relative abundances of
Sprague’s pipit and chestnut-collared longspur. The Model did not converge for vesper
sparrow and McCown’s longspur.
Table 13. Significant songbird community relationships with July ground cover for upland prairie in the
Mankota community pastures and the East Block of Grasslands National Park of Canada, May – July
2008.*
Songbird Community
Variable
Richness
% Shrub Cover
0.1430
0.0616
0.0228
Heterogeneity
% Exposed Bare Ground
0.0287
0.0118
0.0186
% Shrub Cover
0.1098
0.0407
0.0096
Evenness
% Forb Cover
-0.0045
0.0020
0.0270
Horned lark
% Litter Cover
-0.0077
0.0040
0.0600
Sprague’s pipit
% Litter Cover
0.0119
0.0043
0.0076
% Exposed Bare Ground
-0.0233
0.0076
0.0034
% Shrub Cover
-0.0454
0.0263
0.0907
Clay-colored sparrow
% Exposed Bare Ground
0.1092
0.0329
0.0014
Savannah sparrow
% Litter Cover
0.0067
0.0035
0.0574
% Forb Cover
-0.0255
0.0141
0.0774
% Litter Cover
0.0152
0.0055
0.0079
% Standing Dead Cover
0.0246
0.0913
0.0913
% Exposed Bare Ground
-0.0330
0.0094
0.0009
% Exposed Bare Ground
-0.0255
0.0125
0.0472
Baird’s sparrow
Chestnut-collared longspur
Estimate (β)
Standard Error P Value
* Songbird community parameters that were not significantly related to July 2008 ground
cover measurements included the occurrence of McCown’s longspur. The Model did not
converge for vesper sparrow.
60
5.0 Discussion
This is the only study in mixed-grass prairie that provides an assessment of the
burning-grazing interaction effects on songbird diversity and habitat structure. I am aware
of only one other study in mixed-grass prairie of North Dakota that looked at all four
possible treatments with respect to the burning-grazing interaction (i.e., B-G, B-UG, UBG, and UB-UG), however, for this study, the focus was the abundance and nest success of
breeding waterfowl (Kruse and Bowen 1996). Many studies on this interaction have
otherwise been conducted in tall-grass prairies of the United States. Yet, these studies are
of limited use in fire management of the northern mixed-grass prairie because of different
moisture regimes, growing seasons, and species compositions of both plants and animals
(Wright and Bailey 1982). In general, the studies from tall-grass prairies have shown that
grazing increased songbird diversity due to the increased patchiness of grazed grassland,
whereas burning resulted in decreased songbird diversity (e.g., Zimmerman 1997, Powell
2006). Decreased diversity in burned tall-grass prairie, however, can be explained by the
species composition; more species in the songbird community prefer the taller grasses to
the short grasses that appear after burning (Griebel et al. 1998).
In my study, grazing increased richness and heterogeneity when compared to UBUG, although this effect was obvious only in 2007. In both years, the significant burninggrazing interaction indicated that the effects of burning were greater on ungrazed
compared to grazed prairie. The interaction pattern observed in songbird diversity,
however, was generally not reflected in the vegetation structure and ground cover
measurements. For instance, the species richness and heterogeneity in B-G and UB-G
61
was similar, despite the differences in many of the vegetation structure and ground cover
measurements between these treatment groups. In contrast, the difference in species
richness and heterogeneity between B-UG and UB-UG was consistent with a large
difference in the vegetation structure and ground cover measurements. Thus, the disparity
in burning effect size between grazed and ungrazed prairie on songbird diversity may not
be driven entirely by differences in vegetation structure. However, the vegetation
parameters, regardless of treatment, showed significant relationships with the diversity
and abundance of songbirds. This suggests that the relationship between the songbird
community and the vegetation measurements may be a more important factor in
determining habitat suitability than the treatment type or burning-grazing interaction.
As predicted, most vegetation measurements in burned prairie were significantly
different from the unburned prairie. In addition, the vegetation in unburned prairie was
similar regardless of grazing regime, suggesting that grazing had a much smaller effect
than burning. Similarly, the vegetation structure and ground cover was comparable
between B-G and B-UG. This is contrary to the expectation that B-G would have
generally shorter and sparser vegetation than B-UG, due to increased disturbance from
preferential grazing by cattle (Danley et al. 2004). In my study, it is possible that
preferential grazing pressure was low in the burned prairie because the wildfires occurred
at the end of summer. Biondini et al. (1999) looked at grazing by bison (Bison bison)
rather than cattle, but found that vegetation burned in late summer did not regrow
sufficiently during the winter to attract grazers to the same extent as spring fires. In
addition, Erichsen-Arychuk et al. (2002) suggested that summer fires can be detrimental
to the vegetation, because plants are actively growing at the time, and lead to water
62
stresses in the winter months due to reduced snow trapping. Also, perhaps two years postburn is not enough time to detect additive effects of grazing on burned prairie. This may
be because the Mankota pastures have a light- to mid-intensity grazing regime, which
might not be heavy enough to add to the effect of burning during the first two years postburn. Generally, burned-grazed prairie is expected to regenerate more slowly than a burn
where grazing is excluded, because grazing maintains the vegetation at a low stature
(Fuhlendorf et al. 2006). Therefore, the vegetation structure between B-G and B-UG may
become less similar with time.
5.1 Species Richness and Heterogeneity
My results suggest that applying some type of defoliation technique on northern
mixed-grass prairie increases species richness and heterogeneity of the songbird
community. Previous studies in fescue and mixed-grass prairie also have found that
songbird richness is increased after defoliation, whether by mowing, grazing, or fire
(Owen and Myers 1973, Pylypec 1991, Johnson 1997, Madden et al. 1999, Madden et al.
2000, Danley et al. 2004). These patterns may be explained by preferences of most
endemic grassland birds for shorter vegetation, less litter, and more exposed bare ground,
which reflects their evolution in the Great Plains along with frequent disturbances that
historically generated early successional vegetation structures (Mengel 1970, Knopf
1996, Knopf and Samson 1997, Brawn et al. 2001, Fuhlendorf et al. 2006). Likewise, I
found that species richness was negatively associated with litter cover, heterogeneity was
negatively associated with litter depth, and both were positively associated with exposed
bare ground; all factors that were influenced by burning. Additionally, the individual
63
species analyses showed that most of the common species observed in the study area
were either positively related to exposed bare ground, which also indicates low ground
cover, or negatively related to litter or visual obstruction, two measurements related to
the density of the vegetation.
Species richness and heterogeneity were also positively related to shrub cover,
which is opposite to what was expected since burning decreased shrub cover. Shrubs are
commonly used for concealing nests and for perching (Wiens 1969, Wiens 1973),
however, most upland prairie birds do not require shrub cover for suitable breeding
habitat. Because most upland songbirds nest on the ground (e.g., all common species
analyzed except for clay-colored sparrow), tuffs of grass or clumps of vegetation may
also be used for sheltering nests (Wiens 1969, Sutter 1997), and perching can also be
done on rocks, cattle dung, or dead forbs that are strong enough to support the weight of
the bird (Wiens 1973, Wheelwright and Rising 1993, Green et al. 2002). Therefore, the
positive association with shrubs may indicate that when shrubs are present, they are used,
although they are not necessary for suitable breeding habitat by most grassland birds.
Richness was also negatively associated with grass cover, which seems somewhat
surprising for a suite of grassland-specialist songbirds. This negative relationship may
have occurred because grass cover itself was correlated with measurements such as litter
cover and visual obstruction, which most birds were also negatively related to. In
contrast, previous studies have shown that birds are positively related to live vegetation,
which includes both grass and forbs (Sutter 1997, Madden et al. 2000, Green et al. 2002,
Fritcher et al. 2004). My results also indicate a positive association between songbirds
and forb cover. Forbs are often used by ground-nesting songbirds as shelter for nests
64
(Wheelwright and Rising 1993, Lanyon 1994, Hill and Gould 1997). The presence of
forbs probably became more important to birds for nest shelter in the burned prairie
because of the lower vegetation density and relatively barren habitat. Therefore, although
burned prairie had less litter and more bare ground than unburned prairie, suitable nesting
sites were still available. This may clarify the high diversity and abundance of many
species in burned prairie because suitable nest sites are one of the main determinants of
habitat selection by these songbirds (Fondell and Ball 2004).
5.2 Species Evenness
The prairie in B-G had one of the highest levels of species richness and
heterogeneity, although in 2008, the heterogeneity was significantly lower than B-UG
and became more similar to UB-UG. In addition, B-G also had the lowest evenness in
2008, which indicates that one or a few species is predominant in B-G, while others are
more rare. Because the relative abundance of chestnut-collared longspurs was highest in
B-G and much greater than the other four most common species (horned lark, Sprague’s
pipit, Savannah sparrow, Baird’s sparrow), this may indicate predominance by chestnutcollared longspurs and explain the low evenness in this treatment group. Chestnutcollared longspurs were also the predominant species in B-UG and UB-G, yet the
difference in abundance between this species and others was much lower. The low
evenness in B-G may explain the lower heterogeneity in B-G compared to B-UG, since
the heterogeneity calculations are based on the number of individuals in each species
(Peet 1974).
65
Species evenness was affected by treatment in 2008 only, but may be explained
further by the positive association with litter and grass cover. The habitat associations
observed with species richness and heterogeneity may have driven this relationship, since
fewer species were observed with increased litter cover and grass cover. With fewer of
the less common species present in areas of high litter and grass cover, the proportions of
each species are likely to become more uniform (Tokeshi 1993, Wilsey 2005), resulting
in increased evenness of species distributions.
5.3 Individual Species Analyses
The habitat associations of the species analyzed agree with previous assessments
conducted in northern prairies (e.g., Owens and Myers 1973, Stewart 1975, Pylypec
1991, Davis and Duncan 1999, Madden et al. 1999, 2000). In general, the habitat
relationships due to burning or grazing were more evident at the species level rather than
at the scale of species diversity. However, similar conclusions were reached, such that
most grasslands birds have strong affinities to vegetation with low- to mid-densities and
ground cover. Of the eight species analyzed, six were either positively related to exposed
bare ground (horned lark, clay-colored sparrow, vesper sparrow, chestnut-collared
longspur), or negatively related to litter or visual obstruction (horned lark, vesper
sparrow, Savannah sparrow, McCown’s longspur, chestnut-collared longspur).
These habitat associations are consistent with the nesting and foraging habitat
preferences of horned lark (Beason 1995), McCown’s longspur (With 1994), and
chestnut-collared longspur (Hill and Gould 1997). The short and sparse vegetation and
litter depths preferred by these species (Davis and Duncan 1999, Madden et al. 2000,
66
Fritcher et al. 2004), are typically found in burned (Cody 1985, Johnson 1997) or grazed
prairie (Stewart 1975, Davis et al. 1999, Danley et al. 2004). In addition, horned lark and
chestnut-collared longspurs were also positively related to forb cover, and chestnutcollared longspurs were positively related to vegetation height in 2007. These habitat
relationships may seem contrary to the short and sparse vegetation typically associated
with these species (Beason 1995, Hill and Gould 1997); however, forbs and clumps of
tall vegetation are often used to conceal nests (Beason 1995, Hill and Gould 1997, Davis
et al. 1999). Another important factor in habitat selection by McCown’s longspurs is
topography, specifically the presence of hills with south-facing slopes, which are warmer
and available for nesting sooner than other areas of the prairies (With 1994). In addition,
south-facing slopes are preferred by grasshoppers for oviposition sites, which in turn
increase the abundance of invertebrate prey required to feed young (With 1994). This
species was not affected by treatment, thus, areas of hilly grassland where litter and grass
cover is low, even on ungrazed mixed-grass prairies, provides suitable habitat for
McCown’s longspurs.
Low ground cover and vegetation density were also significant habitat
associations of clay-colored sparrow, vesper sparrow, and Savannah sparrow. Claycolored sparrow was related to an increase in the percentage of exposed bare ground,
vesper sparrow was related to low litter and grass cover and high bare ground, and
Savannah sparrow was related to low litter depth, all of which may be explained by the
preference of open areas with short and sparse vegetation for foraging by these species
(Wheelwright and Rising 1993, Knapton 1994, Jones and Cornely 2002). Additional
habitat relationships by vesper sparrow included high shrub and forb cover, which is
67
consistent with previous observations that this species prefers vegetation that is sparse
and where scattered shrubs or clumps of grasses exist (Wiens 1969, Jones and Cornely
2002). Lastly, Savannah sparrow was also related to increased vegetation height, which
may correspond with emergent shrubs and forbs used for perching (Wheelwright and
Rising 1993).
The other two species analyzed, Sprague’s pipit and Baird’s sparrow, had habitat
associations that were generally opposite of the other six (i.e., negatively related to
exposed bare ground and positively related to litter depth and cover), which correspond
with their nesting and foraging habitat preferences. Both species prefer to nest in dense
vegetation with little bare ground (Sutter 1997, Green et al. 2002). In addition, most nests
are covered by dead vegetation from thick clumps of grasses growing nearby (Sutter
1997, Robbins and Dale 1999, Green et al. 2002). Preferred foraging habitat of these
species includes tall vegetation as well, although Baird’s sparrow may prefer the presence
of dense ground cover and litter (Green et al. 2002) more so than Sprague’s pipit
(Robbins and Dale 1999).
While all species analyzed showed some correlation with habitat structure, only
four species were significantly influenced by treatment (horned lark, Sprague’s pipit,
Baird’s sparrow, and chestnut-collared longspur). During the first year post-burn, horned
larks and chestnut-collared longspurs were positively affected by burning and their
abundances were lower in both UB-G and UB-UG, consistent with their habitat
preferences. By the second year post-burn, chestnut-collared longspur abundance in the
burned and grazed prairies was equally high, and the horned lark abundance was no
longer significantly affected by treatment. Considering the habitat preferences for short
68
and sparse vegetation, these transitions in abundances between years may indicate that
burned prairie became more similar to unburned prairie for these species.
In contrast, Sprague’s pipit and Baird’s sparrow were negatively affected by
burning during both years post-burn, with UB-G and UB-UG being preferred equally,
again, consistent with their habitat preferences. These species also declined during the
first couple of years post-burn in the mixed-grass prairies of North Dakota (Johnson
1997, Madden et al. 1999, Danley et al. 2004). Previous studies have shown that
Sprague’s pipit and Baird’s sparrow abundance is related to vegetation heights, densities,
and litter available in both lightly grazed or ungrazed prairie in southern Saskatchewan
(Stewart 1975, Johnson 1997, Davis et al. 1999, Madden et al. 2000). Nevertheless it is
important to note that both species occurred in most of the burned plots in both years,
which indicates that these species can still take advantage of the burned prairie vegetation
structure, although it is not preferred.
5.4 Additional Species
Western meadowlarks and brown-headed cowbirds had low abundances and
occurrences during the point-count surveys and were not analyzed statistically. However,
the pattern observed with these species in the four treatment groups warrants a qualitative
discussion.
Western meadowlarks occurred infrequently in all treatment groups for both
years, although they were one of the most common birds seen in the entire study area.
Most western meadowlarks were recorded outside of the 100-m radius for the point-count
plots and, therefore, were not included in the analyses. This species is reportedly more
69
sensitive to the presence of humans in or near their territories compared to other species
of grassland songbirds (Lanyon 1994), which may explain the low numbers observed
inside the 100-m radius of where the observer was standing. However, the high
abundance of western meadowlarks within the study area is typical of most mixed-grass
prairies (Johnson 1997, Madden et al. 1999, Madden et al. 2000, Danley et al. 2004). In
addition, burning and grazing did not appear to influence their occurrence, probably
because this species has been considered a habitat generalist in grasslands (Cody 1985,
Johnson 1997, Fritcher 2004). Western meadowlarks have demonstrated a preference for
tall and dense vegetation at nest sites to provide nest concealment, substrate for nest
canopies, and sometimes substrate for elaborate tunnels (Lanyon 1994, Davis 2005). The
presence of western meadowlarks in the burned prairie may suggest that, although
vegetation density was low, there was adequate standing dead cover and litter build up
suitable for nest making.
Brown-headed cowbirds are at relatively low densities in the contiguous grassland
regions of southern Saskatchewan, but had greater occurrences in the burned prairies than
the unburned prairies in both years. This species has often been considered a habitat
generalist (Madden et al. 1999, 2000), and as a brood parasite, cues for habitat selection
appear more related to the presence of host birds and nests, than on the structure of the
vegetation (Lowther 1993, Johnson 1997). Therefore, the high occurrence of brownheaded cowbirds in the burned plots may be due to the high number of species and
songbird abundances. In addition, this species prefers foraging sites located in low
ground cover where insects are easily seen (Lowther 1993), and are associated with
ungulates, which agitate the soil and vegetation, making invertebrate prey more
70
accessible (Lowther 1993). The low ground cover and increased productivity and
palatable forage in burned prairie, which may have attracted more wild ungulates (such as
pronghorn [Antilocapra americana] and mule deer [Odocoileus hemionus]) compared to
unburned prairie, thereby increased the forage potential for brown-headed cowbirds. Nest
parasitism by brown-headed cowbirds can reduce the productivity of some prairie
songbirds, including horned lark (Beason 1995), clay-colored sparrow (Knapton 1994),
vesper sparrow (Jones and Cornely 2002), Savannah sparrow (Wheelwright and Rising
1993), Baird’s sparrows (Davis and Sealy 1998, Green et al. 2002), and western
meadowlark (Lanyon 1994). This makes the profusion of brown-headed cowbirds an
important management concern. The implications of increased brown-headed cowbird
abundance in the burned prairie represent a relevant branch for future study.
5.5 Habitat Structure
Lower vegetation, visual obstruction, and litter depths in burned compared to
unburned prairie in 2007 were expected (Madden et al. 1999). After the second year postburn, in 2008, the difference between burned and unburned prairie was no longer as
distinct with respect to these three structural measurements. For instance, maximum
vegetation height of burned prairie became more similar to the unburned prairie two
years after the wildfires, and the effect of burning on visual obstruction and litter depth
after two years was greater in ungrazed prairie than grazed prairie.
Despite the greater similarities of vegetation height, visual obstruction, and litter
depth between burned and unburned prairie, the ground cover measurements such as litter
cover, grass cover, and exposed bare ground showed a distinct dissimilarity between
71
burned and unburned prairie two years post-burn. The differences in ground cover
measurements are characteristic of the vegetation that is left after a grassland fire: high
bare ground, live vegetation, and no litter (P. Fargey, personal communication, June 10,
2009, Species at Risk Specialist with Parks Canada). Grassland fires commonly burn at a
low- to mid-intensity and are hot enough to remove dead matter and litter, but generally
do not to kill live herbaceous vegetation, due to the location of meristems below the soil
(P. Fargey, personal communication, June 10, 2009, Species at Risk Specialist with Parks
Canada). As a result, live vegetation recovers quickly after a burn, the structure of the
habitat becomes less dense with the removal of litter, and the amount of bare ground
increases. The wildfires in my study area burned mostly at a low- to mid-intensity,
however, mid- to high-intensity burns occurred where shrubs were located (P. Fargey,
personal communication, June 10, 2009, Species at Risk Specialist with Parks Canada).
Fires are hotter around shrubs, because of the woody fuel available, and as a result,
usually kill the shrubs and herbaceous vegetation surrounding the shrubs (Bailey 1988).
Burning opened up the ground cover, by exposing bare ground and removing
litter, and therefore provided good foraging habitat to many songbird species (Wiens
1969, Wiens 1973, Wheelwright and Rising 1993, Knapton 1994, Jones and Cornely
2002). In spite of the high bare ground component in burned prairie, live vegetation and
standing dead vegetation was still present. While forb and standing dead cover generally
did not change between treatments, the presence of these cover types was probably very
important in the burned prairie because of the shelter and nesting material they provided
(Wiens 1969, Lanyon 1994, Sutter 1997, Green et al. 2002, Davis 2005). Therefore, in
72
addition to the increased foraging opportunities, suitable nesting sites were also available
in the burned prairie for the grassland songbirds considered in this study.
5.6 Habitat Associations for May versus July 2008
The songbird community (both diversity measurements and abundances of
individual species) was associated with more vegetation structure parameters from the
habitat measured in May, than those measured in July. Starting in May, or earlier in
March/April for some species (e.g., horned lark [Stewart 1975]), habitat selection begins
for breeding birds. It is most likely for this reason that more habitat relationships exist
between the birds and May vegetation structure, rather than later in the summer. When
possible, vegetation data should be collected at the same time as the point-count surveys,
if the objective is to examine the habitat associations of the birds.
Although fewer vegetation relationships existed from the habitat measured in
July, the effect of vegetation structure on the songbird community was generally
consistent with the vegetation relationships observed with the May vegetation.
Furthermore, the habitat associations of individual species from both months are
consistent with the grassland songbird habitat affinities observed in previous studies. This
demonstrates that while a greater number of vegetation relationships will be captured if
using measurements from May, July vegetation data collection is still viable.
73
6.0 Conclusions and Recommendations
These results suggest a conservation strategy for songbird diversity involving
grassland burning. Burning may be used on ungrazed mixed-grass prairie of southern
Saskatchewan where the goal of grassland management is to maintain high levels of
songbird richness and heterogeneity. On mixed-grass prairie with a light- to mid-intensity
grazing regime, burning does not affect songbird richness and heterogeneity. Further, low
2008 heterogeneity levels in B-G may suggest that burning on grazed fields is not only
unnecessary, but possibly detrimental to songbird diversity. This was explained, however,
by the predominance of chestnut-collared longspurs in this treatment group, rather than
the vegetation structure. In addition the species richness and abundances of individual
species were not significantly different between B-G and B-UG, therefore, whether the
combination of burning and grazing is actually detrimental to the songbird community is
unclear.
While burning increased the diversity of the songbird community, it had a
detrimental effect on the abundances of Sprague’s pipit and Baird’s sparrow. It is
important to recognize that some species do not respond positively to burning, at least
during the first two years post-burn, revealing a need to maintain habitats in a variety of
successional stages. These observations support work from previous studies in North
Dakota that found most species respond positively to fire, although for Sprague’s pipit
and Baird’s sparrow, this positive response does not occur until three to five years postburn (Johnson 1997, Madden et al. 1999, Danley 2004). Continued songbird surveys of
74
the burned prairie beyond two years post-burn are required to confirm a similar pattern
and time period in Saskatchewan mixed-grass prairie.
The structural components of vegetation generally decreased as a result of
burning and grazing, yet different levels of litter cover and exposed bare ground were still
generated by the two disturbances. Because the burning or grazing disturbances increased
songbird richness and heterogeneity (grazing only in 2007), this suggests that species
richness and heterogeneity are high in vegetation with low- to mid-densities and ground
cover, but drop once vegetation densities and ground cover become as great as that of
unburned-ungrazed prairie. Strong associations with such habitat can be explained by the
evolution of these birds to frequent grassland disturbances that historically generated
early successional vegetation structures (Mengel 1970, Knopf 1996, Knopf and Samson
1997, Brawn et al. 2001, Fuhlendorf et al. 2006). The lower species diversity in
unburned-ungrazed prairie indicates that the taller and denser later seral vegetation was
unattractive habitat to many grassland birds (Madden et al. 1999, Askins et al. 2007).
The vegetation structure and abundances of individual species on B-G was similar
to that of B-UG, at least during the first two years post-burn. It is expected that a
disparity between B-G and B-UG will start to appear as the vegetation progresses toward
unburned prairie. At this point, however, the time frame of successional changes in
vegetation structure and the songbird community in Saskatchewan’s mixed-grass prairie
has not been determined. Research from North Dakota suggests a natural fire interval in
northern mixed-grass prairie at five to 10 years (Wright and Bailey 1982, Madden et al.
1999). In addition, studies from North Dakota have determined that burned vegetation
became more similar to unburned prairie after two years post-burn (Madden et al. 2000),
75
and songbird diversity and habitat quality flourished three to five years post-burn
(Johnson 1997, Madden et al. 1999, Danley et al. 2004). While these studies provide
important hypotheses and useful knowledge, they are from a more mesic ecosystem
(Wright and Bailey 1982) and might not be transferable to the more arid prairies of
southern Saskatchewan. Therefore, further study on the long-term effects and
successional changes in northern mixed-grass prairie of southern Saskatchewan are
recommended.
I encourage GNPC to consider burning in their management plans in light of
conservation prospects of grassland songbird diversity. However, when considering fire
as a management tool, it is imperative to understand the public’s opinion and value of
reintroducing fire onto the prairie landscape. Fire is often perceived as a threat among
most rural communities, and as such, the surrounding communities of GNPC are for the
most part opposed to prescribe burning because of risks to homes and livelihoods. No
final decision on a fire management plan should be made until the surrounding
communities and landowners themselves support that decision. Therefore, prescribed
burning programs need to be implemented slowly and in response to the wishes of the
public. When burning is not socially permissible, it is important to establish good
communication with neighbouring landowners, such as transparent planning, being
responsive to landowner’s fears, and providing education, which builds trust between
managers and landowners, enabling the public to be more supportive of prescribed
burning initiatives.
I also encourage further research on the effects of fire in northern mixed-grass
prairie before implementing burning into park management plans. In addition to the study
76
of successional changes, other research avenues include the effects of fire on individual
species’ vital rates, which include factors such as nest success, brood parasitism,
predation risk, food quality, or biomass. Without this information in burned prairie we
cannot predict for certain the impacts of burning on songbird populations and whether
burning provides habitat of higher quality, or provides an ecological sink (Vickery et al.
1992, Leuders et al. 2006). Another branch of future study involves landscape scale
influences of burning on the songbird community. Because fire is a landscape scale
process, the size and spatial pattern of burned patches may influence the songbird
community at a much broader scale than that addressed in my study. Burning can be a
form of habitat fragmentation, introduce edges onto the prairie landscape, and generate
distinct habitat patches. While previous studies have demonstrated that landscape factors
such as habitat fragmentation, amount of edge, and patch size can influence habitat
selection by grassland songbirds (Davis 2004, Koper and Schmiegelow 2006, Van Dyke
et al. 2007), the effects of edge and area sensitivity on grassland songbirds in response to
burned prairie is absent from published literature.
77
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Appendices
Appendix I. Songbird species included in 2007 data analysis and fly-bys removed.
Fly-bys removed
Species Included in Project
Species
Quantity
Treatment Group (site)
Horned Lark
(Eremophila alpestris)
Eastern Kingbird
(Tyrannus tyrannus)
2
Unburned-Ungrazed (pasture 4)
Sprague's Pipit
(Anthus spragueii)
Lark Bunting
(Calamospiza melanocorys)
1
Burned-Grazed (burn 5)
Clay-colored Sparrow
(Spizella pallida)
Red-winged Blackbird
(Agelaius phoeniceus)
1
Burned-Grazed (burn 8)
Brewer's Sparrow
(Spizella breweri)
Vesper Sparrow
(Pooecetes gramineus)
Savannah Sparrow
(Passerculus sandwichensis)
Baird's Sparrow
(Ammodramus bairdii)
McCown's Longspur
(Calcarius mccownii)
Chestnut-collared Longspur
(Calcarius ornatus)
Western Meadowlark
(Sturnella neglecta)
Brown-headed Cowbird
(Molothrus ater)
85
Appendix II. Songbird species included in 2008 data analysis and fly-bys removed.
Fly-bys removed
Species Included in Project
Species
Quantity
Treatment Group (site)
Horned Lark
Eastern Kingbird
1
Burned-Ungrazed (burn 4)
Cliff Swallow
(Petrochelidon pyrrhonota)
Eastern Kingbird
2
Unburned-Grazed (pasture 12)
Barn Swallow
(Hirundo rustica)
Lark Bunting
5
Unburned-Ungrazed (pasture 3)
Sprague’s Pipit
Red-winged Blackbird
1
Burned-Grazed (burn 5)
Clay-colored Sparrow
Red-winged Blackbird
1
Burned-Grazed (burn 8)
Brewer's Sparrow
Red-winged Blackbird
1
Unburned-Grazed (pasture 13)
Vesper Sparrow
American Goldfinch
5
Burned-Grazed (burn 8)
Lark Bunting
Savannah Sparrow
Grasshopper Sparrow
(Ammodramus savannarum)
Baird's Sparrow
McCown's Longspur
Chestnut-collared Longspur
Western Meadowlark
Brewer's Blackbird
(Euphagus cyanocephalus)
Brown-headed Cowbird
American Goldfinch
(Carduelis tristis)
86
Appendix III. Means and standard errors for the songbird community per plot (3.2 ha) in
the four treatment groups; burned-grazed (B-G), burned-ungrazed (B-UG), unburnedgrazed (UB-G), and unburned-ungrazed (UB-UG), June 2007.
Songbird Community
B-G (n = 16)
B-UG (n = 16)
UB-G (n = 21)
UB-UG (n = 53)
Richness
3.75 ± 0.25
4.25 ± 0.36
3.95 ± 0.21
3.26 ± 0.13
Heterogeneity
3.33 ± 0.21
3.85 ± 0.34
3.56 ± 0.12
2.98 ± 0.11
Evenness
0.90 ± 0.02
0.93 ± 0.01
0.92 ± 0.01
0.93 ± 0.01
Horned lark
1.06 ± 0.28
1.13 ± 0.18
0.95 ± 0.21
0.25 ± 0.08
Sprague's pipit
0.69 ± 0.18
0.81 ± 0.16
1.81 ± 0.16
1.83 ± 0.14
Savannah sparrow
0.63 ± 0.24
0.38 ± 0.15
0.57 ± 0.19
0.75 ± 0.12
Baird's sparrow
0.94 ± 0.28
1.13 ± 0.29
2.10 ± 0.24
2.19 ± 0.15
Chestnut-collared longspur
3.06 ± 0.42
2.25 ± 0.38
1.57 ± 0.31
0.28 ± 0.08
Diversity
(# species per plot)
Relative Abundance
(# individuals per plot)
Appendix IV. Means and standard errors for the songbird community per plot (3.2 ha) in
the four treatment groups; burned-grazed (B-G), burned-ungrazed (B-UG), unburnedgrazed (UB-G), and unburned-ungrazed (UB-UG), May – June 2008. Occurrences are
percentage of plots in which the species is present.
Songbird Community
B-G (n = 16)
B-UG (n = 16)
UB-G (n = 21)
UB-UG (n = 53)
Richness
5.38 ± 0.34
6.25 ± 0.28
5.29 ± 0.23
4.68 ± 0.21
Heterogeneity
4.13 ± 0.28
5.18 ± 0.27
4.27 ± 0.17
3.71 ± 0.15
Evenness
0.84 ± 0.02
0.89 ± 0.01
0.87 ± 0.01
0.86 ± 0.01
Horned lark
1.06 ± 0.20
0.58 ± 0.14
0.63 ± 0.11
0.23 ± 0.06
Sprague's pipit
0.73 ± 0.18
1.10 ± 0.17
1.60 ± 0.15
1.81 ± 0.07
Savannah sparrow
0.54 ± 0.20
0.77 ± 0.11
0.30 ± 0.09
0.33 ± 0.05
Baird's sparrow
1.40 ± 0.36
1.65 ± 0.21
2.24 ± 0.19
2.72 ± 0.09
Chestnut-collared longspur
2.96 ± 0.33
2.04 ± 0.26
2.29 ± 0.21
0.57 ± 0.08
Clay-colored sparrow
18.75
18.75
9.52
20.75
Vesper sparrow
43.75
62.50
9.52
20.75
McCown's longspur
31.25
25.00
33.33
5.66
Diversity
(# species per plot)
Relative Abundance
(# individuals per plot)
Occurrence
(% of plots where present)
87
Appendix V. Means and standard errors for the vegetation measurements per plot (3.2
ha), in the four treatment groups; burned-grazed (B-G), burned-ungrazed (B-UG),
unburned-grazed (UB-G), and unburned-ungrazed (UB-UG), July 2007.
Vegetation Measurement
B-G (n = 16)
B-UG (n = 16)
UB-G (n = 21)
UB-UG (n = 53)
Maximum Vegetation Height (cm)
42.59 ± 0.72
42.61 ± 1.09
47.17 ± 1.27
50.80 ± 1.19
Visual Obstruction Reading (cm)
4.30 ± 0.23
4.38 ± 0.26
10.74 ± 0.81
8.86 ± 0.55
Litter Depth (cm)
1.05 ± 0.05
1.03 ± 0.06
7.86 ± 0.62
6.94 ± 0.46
Appendix VI. Means and standard errors for the vegetation measurements per plot (3.2
ha) in the four treatment groups; burned-grazed (B-G), burned-ungrazed (B-UG),
unburned-grazed (UB-G), and unburned-ungrazed (UB-UG), May 2008.
Vegetation Measurement
B-G (n = 16)
B-UG (n = 16)
UB-G (n = 21)
UB-UG (n = 53)
Maximum Vegetation Height (cm)
37.23 ± 1.69
39.41 ± 1.30
41.39 ± 0.75
41.69 ± 0.70
Visual Obstruction Reading (cm)
3.48 ± 0.70
3.99 ± 0.36
3.36 ± 0.42
7.23 ± 0.30
Litter Depth (cm)
1.64 ± 0.22
1.14 ± 0.15
2.06 ± 0.26
5.09 ± 0.40
Litter Cover (%)
14.91 ± 1.95
10.38 ± 1.64
47.32 ± 4.13
62.75 ± 2.20
Standing Dead Cover (%)
8.81 ± 1.29
11.28 ± 1.31
9.55 ± 1.78
14.98 ± 0.90
Exposed Bare Ground (%)
19.45 ± 4.11
15.44 ± 4.13
3.24 ± 1.19
1.64 ± 0.51
Grass Cover (%)
30.73 ± 2.25
31.44 ± 2.49
17.28 ± 1.61
14.81 ± 0.77
Forb Cover (%)
6.14 ± 0.93
6.75 ± 1.27
7.76 ± 0.85
5.11 ± 0.36
Shrub Cover (%)
0.66 ± 0.33
0.47 ± 0.24
0.36 ± 0.15
2.10 ± 0.40
88