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Landbird HSI Model Draft • Tirpak and Jones-Farrand
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Bachman’s Sparrow (Aimophila aestivalis)
Status:
The Bachman’s sparrow is a resident bird associated with pine savannahs and open
habitats of the southeastern United States. Although its range expanded north to include
Illinois, Indiana, and Ohio in response to the land clearing at the turn of the 20th century,
its range has consistently contracted over the last 100 years. Today, it is mostly restricted
to the extreme Southeast (Figure 9). Breeding bird survey data from the Central BBS
region indicates significant annual declines (8.1%) over the past 40 years; declines have
been particularly pronounced since 1980 (20.8% per year). The species is a USFWS Bird
of Conservation Concern in both the CH and WGCP BCRs (Appendix 2). PIF classifies
this species as a Tier I priority with a regional combined score of 20 in both BCRs
(Appendix 3).
Figure 9. Relative abundance of Bachman’s sparrows, derived from Breeding Bird
Survey data, 1994 – 2003.
Natural History:
Bachman’s sparrows occupy two main habitats in the Southeast: mature (>80 year old)
pine stands that are frequently burned (<3 year burn interval) and recent (<5 year old)
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clearcuts, powerline rights-of-way, and dry grasslands (Dunning and Watts 1990).
However, productivity is lower in these latter habitats (1 vs. 3 offspring/pair/year; Liu et
al. 1995, Perkins et al. 2003). Based on this and the poor colonizing ability of this
species – suitable clearcut habitats >3 km from a source population generally remain
unoccupied in South Carolina (Dunning et al. 1995) – Tucker et al. (2004) considered
Bachman’s sparrow to be nearly endemic to mature longleaf pine stands.
In all studies of Bachman’s sparrow, two critical microhabitat features are repeatedly
observed as important within the birds’ 2.5 ha territories (Haggerty 1998) - dense grass
understories and open canopies, both of which are maintained through frequent fires
(Haggerty 1998, Plentovich et al. 1998, Tucker et al. 2004, Wood et al. 2004). Typically,
stands managed for red-cockaded woodpeckers provide habitat for Bachman’s sparrows,
as fires occur frequently enough to suppress dense woody understories (Wilson et al.
1995, Plentovich et al. 1998, Provencher et al. 2002, Wood et al. 2004).
Model Description:
Our habitat suitability model for Bachman’s sparrows contains six parameters related to
relative density (landform, landcover type, age class, forest patch size, canopy cover, and
connectivity) and two parameters related to productivity (percent forest in the landscape
and edge occurrence).
The first suitability function combined landform, landcover type, and age class into a
single matrix (SI1) that defined unique combinations of these classes. We then directly
assigned relative density values to these combinations based on habitat suitability data
from Hamel (1992; Table 11).
Forest patch size (SI2) was also included as a factor because of the relatively large size of
home ranges for this species (up to 5.1 ha). Home range size varied among regions and
habitat types (reviewed in Mitchell 1998), being slightly larger in evergreen stands (4.8
ha) than in ephemeral early successional habitats (2.2 ha). We used these values as
minimum threshold sizes of sparrow presence in each of these habitat types (Table 12).
We included canopy cover (SI3) as a fourth parameter to satisfy the two-fold requirement
for open canopies and dense understories, two habitat components often well-correlated,
particularly at the low end of the canopy closure spectrum (Table 13). Haggerty (1998)
observed 9.5% canopy closure at sites occupied by Bachman’s sparrows and 40% at
unoccupied sites. Wood et al. (2004) observed 20 times more Bachman’s sparrows in
habitats with 25-50% canopy cover versus sites with 50-75% canopy cover. We fit an
inverse logistic function to these data to extrapolate values between these known points
(Figure 10).
As a resident species relatively restricted to a specialized habitat, the occupancy of a
habitat by Bachman’s sparrows is affected by the ability of dispersers to colonize
unoccupied areas. This ability is, in turn, limited by the connectivity of habitat patches
(SI4). Dunning et al. (1995) found that birds were unable to colonize clearcut habitats
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further than 3 km from a resident population before succession made the clearcut
unsuitable. Isolation of mature evergreen stands is also an issue for Bachman’s sparrows;
however, because the conditions within them are less ephemeral, birds have a longer time
to access them. Thus, they can likely be more isolated and still be colonized. We
assumed the stability of these habitat conditions increased their effective isolation
distance to 9 km (Table 14; Figure 11).
To calculate relative density, the maximum value from the landcover type and forest age
class matrix must first be identified. From this, we used the appropriate patch size
threshold to remove patches too small to contain a home range. Next, we multiplied all
pixels by the appropriate value for canopy cover. Lastly, we assessed the potential
colonization of a pixel by determining the distance between each habitat patch and its
nearest neighbor and applying the appropriate equations for the habitat type.
Relative density = (SI1 * SI2 * SI3 * SI4)0.250
Relative productivity was a function of two factors: the proportion of forest in the
landscape (10 km radius buffer; SI5a) and presence of edge (SI5b). We defined the
functional relationship between landscape composition, edge, and relative productivity
based on data from Robinson et al. (1995) and Donovan et al. (1997) and the
susceptibility of each species to cowbird parasitism (Figure 12, Table 15).
Relative productivity = SI5
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SI1: Landform, landcover type, and age class
Table 11. Relationship between landform, landcover type, age class, and relative productivity of
Bachman’s sparrows. Values in parentheses apply to West Gulf Coastal Plain.
.
Successional Age Class
Landform
Landcover type
GrassShrubSapling
Pole
Saw
Forb
Seedling
Floodplain/Valley Low-density Residential
0.000
0.000
0.000
0.000
0.000
Transitional/Shrubland
0.000
0.000
0.000
0.000
0.000
Deciduous
0.000
0.000
0.000
0.000
0.000
Evergreen
0.333
0.333
0.333
1.000
0.000
Mixed
0.333
0.333
0.333
0.000
0.000
Orchard/vineyard
0.000
0.000
0.000
0.000
0.000
Woody Wetlands
0.000
0.000
0.000
0.000
0.000
Terrace/Mesic
Low-density Residential
0.000
0.000
0.000
0.000
0.000
Transitional/Shrubland
0.000
0.333
0.333
0.333
(1.000) 0.000
Deciduous
0.333
0.333
0.333
0.000
0.000
Evergreen
0.333
0.333
0.333
1.000
0.000
Mixed
0.333
0.333
0.333
0.000
0.000
Orchard/vineyard
0.333
0.333
0.333
0.000
0.000
Woody Wetlands
0.000
0.000
0.000
0.000
0.000
Xeric/ridge
Low-density Residential
0.000
0.000
0.000
0.000
0.000
Transitional/Shrubland
0.000
0.000
0.333
0.333
0.333
(1.000) (1.000)
Deciduous
0.333
0.333
0.333
0.000
0.000
Evergreen
1.000
0.000
0.333
0.333
0.333
(1.000) (1.000)
Mixed
0.333
0.333
0.333
0.000
0.000
Orchard/vineyard
0.333
0.333
0.333
0.000
0.000
Woody Wetlands
0.000
0.000
0.000
0.000
0.000
All landforms
Herbaceousa
1.000
0.000
0.000
0.000
0.000
Non-habitatb
0.000
0.000
0.000
0.000
0.000
Includes natural grasslands, pasture/hay, fallow, urban/recreational grasses, emergent herbaceous
wetlands.
b
Includes open water, high intensity residential, commercial/industrial/transportation, bare
rock/sand/clay, quarries/strip mines/gravel pits, row crops, and small grains.
a
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SI2: Forest patch size
Table 12. Relationship between forest patch size (ha) and relative density of
Bachman’s sparrow.
Habitat
Evergreen
Forest patch size (ha)
Relative density
<4.8 a
0.000
≥4.8
1.000
Transitional/shrublands/grasslands/fallow
<2.2 a
0.000
≥2.2
1.000
a
Cut points based on data from Stober 1996, referenced in Mitchell 1998.
SI3: Canopy cover
Table 13. Relationship between canopy cover (%) and relative density of Bachman’s
sparrow.
Canopy cover (%)
0a
9.5 b
37.5 c
62.5 c
100 a
a
Assumed values
b
Haggerty (1998)
c
Wood et al. (2004)
Relative density
1.000
1.000
1.000
0.050
0.000
Landbird HSI Model Draft • Tirpak and Jones-Farrand
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1.2
6
0
1.0
Relative density (SI3)
0
0.8
0
0.6
0
0.4
0
0.2
0
0.0
0
20
40
60
80
100
Canopy cover (%)
Figure 10. Relationship between canopy cover (%) and relative density of Bachman’s
sparrows.
Equation: Relative density = 1 - (1.000 / (1 + (126024970 * e -0.3455 * canopy cover))
SI4: Habitat connectivity
Table 14. Relationship between habitat connectivity (km) and occupancy of habitat by
Bachman’s sparrows.
Habitat
Evergreen
Transitional/shrublands/grasslands/fallow
a
b
Dunning et al. (1995)
Assumed
Habitat connectivity (km)
0a
9b
0a
3a
Occupancy
1.000
0.000
1.000
0.000
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1.00
Relative density
0.80
0.60
0.40
0.20
0.00
0
2
4
6
8
10
Habitat connectivity (km)
Clearcut/Transitional
Mature evergreen
Figure 11. Relationship between habitat connectivity (km) and occupancy of habitat by
Bachman’s sparrows.
Equations:
Clearcut/transitional: Occupancy = -0.3333 * (habitat connectivity) + 1
Mature evergreen: Occupancy = -0.1111 * (habitat connectivity) + 1
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SI5: Generalized productivity function
SI5a: Generalized productivity function (Forest composition)
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Predation and parasitism
Relative productivity
0.75
0.5
0.25
0
0
25
50
75
Landscape composition (% forest in 10 km radius buffer)
Figure 12. Relationship between landscape composition (% forest in 10 km radius
buffer) and relative productivity of Bachman’s sparrows.
Equation: Relative productivity = (0.0076 * (% forest)) + 0.2434
SI5b: Generalized productivity function (Edge)
Table 15. Influence of edge on relative productivity values derived from landscape
composition (% forest) for Bachman’s sparrows.
Forest composition
Edge a
Core
0-20%
Xb
X
20-80%
X * 0.8
X * 1.2 c
80-100%
X
X
a
“Edge” defined as any forest pixels with a nonforest pixel within a 3 x 3 window.
Forest-nonforest classification same as used in SI5a.
b
X = value derived from SI5a
c
Maximum value = 1
100