Download Hunting habitat selection by hen harriers on moorland: Implications

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Hunting habitat selection by hen harriers on moorland:
Implications for conservation management
Beatriz Arroyoa,d,*, Arjun Amarb,c, Fiona Leckiea,e, Graeme M. Buchananc,
Jeremy D. Wilsonc, Stephen Redpatha,f
a
Centre for Ecology and Hydrology (CEH), Hill of Brathens, Banchory AB34 4BW, Scotland, UK
Game Conservancy Trust, c/o CEH Banchory, Hill of Brathens, Banchory AB34 4BW, Scotland, UK
c
Royal Society for the Protection of Birds, Dunedin House, 25 Ravelston Terrace, Edinburgh EH4 3TP, Scotland, UK
d
Instituto de Investigación en Recursos Cinegéticos (IREC) (CSIC-UCLM-JCCM), Ronda de Toledo s/n, 13071 Ciudad Real, Spain
e
Natural Research Ltd., Banchory Business Centre, Burn O’Bennie Road, Banchory, AB31 5ZU, Scotland, UK
f
Dept. Biological Sciences, Aberdeen University, Tillydrone Rd., Aberdeen, Scotland, UK
b
A R T I C L E I N F O
A B S T R A C T
Article history:
We examine habitat use by hunting hen harriers Circus cyaneus at three study sites in Scot-
Received 5 August 2008
land to evaluate whether foraging patterns differ between sexes, sites, and stages of the
Received in revised form
breeding period. We modelled time spent hunting in focal plots as a function of habitat
22 November 2008
and nest proximity. Male hunting intensity (time spent hunting per hour of observation
Accepted 23 November 2008
and km2) varied between sites and breeding periods, being lower during the nestling than
Available online 3 January 2009
the incubation period. Habitat use patterns were mostly consistent among study sites,
which is important for developing species management recommendations applicable over
Keywords:
the species’ range. Males avoided improved grassland, and selected areas of mixed heather
Habitat management
and rough grass (with an optimum at ca. 50% heather cover). The effect of nest proximity
Protected areas
was small. In contrast, females hunted mainly within 300–500 m of the nest, with a small
Foraging ecology
additive effect of vegetation cover, areas of fragmented heather being preferred. Habitat
Modelling
management to benefit foraging harriers will involve creating (or maintaining) mosaics
of heather/grassland around nest areas. Additionally, it might be possible to manipulate
habitat to reduce conflict in areas where harrier predation on red grouse is important by
segregating areas holding highest grouse densities (with high heather cover) from those
favoured for harrier foraging (heather–grass mosaics). However, it would be necessary to
test whether these manipulations might also influence harrier nest distribution, an effect
which could negate any benefits from this strategy.
Ó 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Conservation of wild bird species is frequently delivered
through management of their habitats, and this is recognised
in legislation. For example, the European Union Directive on
the conservation of wild birds (79/409/EEC) requires the mem-
ber states to identify and classify the most suitable areas for
certain rare or vulnerable species as special protection areas
(SPAs). These are intended to safeguard the habitats of the
species for which they are selected. In order to maintain or
improve the suitability of these areas for the target species,
information about habitat preferences is required (Rouquette
* Corresponding author: Address: IREC, Ronda de Toledo s/n, 13071 Ciudad Real, Spain. Tel.: +34 926295450; fax: +34 926295451.
E-mail address: [email protected] (B. Arroyo).
0006-3207/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biocon.2008.11.013
B I O L O G I C A L C O N S E RVAT I O N
and Thompson, 2005; Serrano and Astrain, 2005). Many studies focus on habitat selection for nesting (e.g. Newton et al.,
1981; Suarez et al., 2000; Poirazidis et al., 2004). However,
effective management must also consider foraging habitat
selection (e.g. Donazar et al., 1993; Franco and Sutherland,
2004; Martin and Possingham, 2005; Sergio et al., 2006; Garcia
et al., 2006; Hinam and Clair, 2008), which may be different
and occur over different spatial scales (Sergio et al., 2003;
Carrete and Donazar, 2005).
Knowledge of foraging preferences is also useful to help
resolve management conflicts, as for example when protected predators prey on species of conservation or economic
value (Roby et al., 2002; Madders and Walker, 2002).
Preferred foraging habitats will often be those with the
highest prey availability (Tella et al., 1998; Wilson et al.,
2005; Benton et al., 2002). For central-place foragers, the use
of a resource patch within a heterogeneous environment will
be a function of both quality and availability of food resource,
and distance to the central place (Matthiopoulos, 2003). The
relative importance of these two factors affects how a species
uses its environment and therefore, how best to manage the
environment for the species. Furthermore, where there are
sex differences in parental role or sexual dimorphism affecting prey choice, habitat selection may differ between the
sexes, and management recommendations may need to recognise this. Moreover, the extent to which these habitat associations are consistent between locations will determine if
they can be used to inform management across a region or
country, or are site specific (Whittingham et al., 2003, 2007).
The hen harrier Circus cyaneus is a species of conservation
concern in the UK (Gregory et al., 2002), for which several SPAs
have been designated. Hen harriers in the UK nest predominantly in heather Calluna vulgaris moorland (Redpath et al.,
1998; Sim et al., 2007), and all SPAs are in such areas. Previous
research (Amar et al., 2008) has shown that in one SPA, the
proportion of preferred foraging habitat within 2 km of nest
sites is positively related to breeding success. Thus, creation
or maintenance of preferred foraging habitat around nests
might be a worthwhile conservation practice for this species.
However, some heather moorland habitat in the UK is managed for commercial shooting of red grouse Lagopus lagopus
scoticus, a moorland game bird of considerable economic
importance, and in these areas hen harriers are the focus of
a significant human-wildlife conflict. In certain circumstances (when grouse are in low numbers and harriers in high
numbers) predation of grouse chicks by hen harriers to feed
their nestlings can limit the numbers of grouse available for
shooting in the autumn (Redpath and Thirgood, 1999; Thirgood et al., 2000). Quantifying harrier foraging habitat preferences could therefore be useful in the management of such
areas, to design mitigation measures aimed at reducing the
level of predation, although this may be complicated by sexual differences in prey use (Redpath and Thirgood, 1997; Amar
et al., 2004). Such differences may arise because the two sexes
may select different foraging habitats, or because they hunt at
different distances from the nest (Garcı́a and Arroyo, 2005; Arroyo et al., 2006). Moreover, because hen harriers nest in
heather (Redpath et al., 1998), the proportion of this habitat
within an area is likely to be spatially autocorrelated with
proximity to nest sites (Rosenberg and McKelvey, 1999).
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587
Several previous studies have considered the foraging
behaviour of hen harriers, but have not controlled for nest
location or have not differentiated between the sexes, and
have often been based on single study areas (e.g. Madders,
2003; Thirgood et al., 2003; Amar and Redpath, 2005). Additionally, no previous study of this species has considered
whether habitat selection changes through the breeding period. In this study we examine habitat use by hunting male
and female hen harriers in the incubation and nestling periods at three different sites in Scotland, whilst controlling for
nest location. We consider our results in the context of conservation management of hen harriers and the feasibility of
habitat management designed to reduce conflict with management for grouse shooting.
2.
Methods
2.1.
Study sites and observation plots
Data on hunting harriers were collected from three study
sites, all of which include harrier SPAs: Langholm, and the
Glen App and Galloway moors (hereafter termed Galloway)
in southwest Scotland, and Orkney Mainland Moors on the
Orkney Islands, northern Scotland (Fig. 1). A summary of
the data available for the study is shown in Table 1. Four years
of data were collected at Langholm and on Orkney, and 2
years in Galloway. Data for Orkney were collected in 1998–
1999 and 2002–2003, Langholm data were collected between
1994–1996 and 2002, and Galloway data were collected in
2004–2005. At each site we recorded by visual observations
harrier hunting in plots measuring between 0.25 km2 and
46 km2 (depending on visibility) and located within 5 km of
a harrier nest site (Table 1). The location of observation plots
within each study site was selected to cover all ranges of distances to nests (up to 5 km) and habitats (taking into account
access and visibility constraints). The boundaries of some
observation plots could be visualised easily on the ground
through obvious landscape features such as streams or
fences, for others we marked plot boundaries on the ground
with 4ft high corner flags.
Fig. 1 – Location of the study sites.
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Table 1 – Summary of data and characteristics for each study site.
Number of watches
Number of observation plots
Total observation hours
Average (±SD) watch length (min)
Average ± SD observation plot size (km2) (range)
Watch period (range of relative dates)
Number of breeding pairs per study year
Distances from observation plot to nearest nest (km)
NPI range
NPI range (nestling)
Overall proportion (%) of dense heather (range within
observation plots)
Overall proportion (%) of open heather (range within
observation plots)
Overall proportion (%) of rough grass (range within
observation plots)
Correlation between % dense heather and % rough grass
within observation plots (P)
Correlation between % open heather and % rough grass
within observation plots (P)
Galloway
Langholm
Orkney
150
16
414.3
166 ± 25
0.64 ± 0.15 (0.44–0.95)
1 to 78
9.0 ± 1.4
0.3–4.4
0.22–5.79
0.07–3.61
12 (0–77)
357
26
872.9
147 ± 38
0.97 ± 0.29 (0.25–1.45)
1 to 88
10.5 ± 6.0
0.26–4
0.06–30.57
0.06–9.66
15 (0–75)
193
27
435.1
135 ± 38
0.90 ± 0.19 (0.5–1.0)
3 to 97
31.5 ± 8.9
0.17–2.8
0.24–51.18
0.15–28.59
27 (0–99)
15 (1–40)
17 (0–71)
3 (0–22)
32 (5–89)
45 (1–99)
17 (0–62)
0.48 (0.05)
0.81 (0.0001)
0.26 (n.s.)
0.30 (n.s.)
0.83 (0.0001)
0.53 (0.005)
Observations on Orkney took place at 26 plots, eight of
which were watched in 2 years, nine in 3 years and nine in
all 4 years. At Langholm, observations were carried out on
27 plots, 16 of which were watched in only 1 year, four in 2
years and seven in 3 years. Data from Langholm in 1994–
1996 (but not in 2002) were those used in another study
(Thirgood et al., 2003). In Galloway observations were carried
out on 16 plots, of which only 10 were observed in both years.
We calculated an average harrier lay date at each study
site in each year, using data obtained by regular monitoring
and nest visits to all breeding pairs in each year. We defined
‘‘incubation period’’ as the period between the mean lay date
(day 0) and the mean hatch date (day 33) in each year, and
‘‘nestling period’’ as the period after the mean hatch date
(>day 33). Hunting data during the incubation period were
available for Langholm in all years, for Orkney in 2003 and
for Galloway in 2004; data during the nestling period were collected from all sites in all years. Chicks take around 35 days to
fledge, and we watched plots throughout the nestling stage
into the fledging period.
We also calculated a nest site proximity index (NPI) for
each observation plot in each year, as the sum of the reciprocals of the squared distance from the centre of the plot to
each nest site (km) in the study area; this variable has been
used to examine foraging of harriers (Madders, 2003; Thirgood
et al., 2003; Amar and Redpath, 2005), with higher values indicating closer proximity to more nests. Observation plots excluded nests, but because plots vary in size then NPI might
vary with plot size simply because nests can be closer to
the centre of small plots than large ones. However, in reality
NPI was uncorrelated with plot size (r = 0.08, n = 87, P = 0.45
for plots observed during incubation, or r = 0.03, n = 108,
P = 0.71 for plots observed during nestling). NPI during the
incubation period included all nests that produced clutches,
while NPI during the nestling period was calculated using
only nests with hatched clutches.
2.2.
Quantification of harrier hunting
Watches for hunting harriers were undertaken between
06.00 h and 21.00 h, and each lasted for 2–3 h each (Table
1). Each observation plot was observed every few days
(6.7 ± 6), and watches in each plot were spread out throughout the day. We only conducted watches when there was little or no rain. During a watch, observation plots were
scanned continuously through binoculars for hunting harriers. When an individual harrier was seen hunting in an
observation plot we recorded its sex and the time it spent
hunting within the observation plot (s). Harriers were classified as hunting if they were flying <10 m above the ground
(estimated visually) as described by Amar and Redpath
(2005) and Madders (2003). As harriers tend to hunt by flying
low and quartering the ground (Schipper, 1977), a height of
10 m was chosen to exclude harriers that may not have been
actively searching for prey. Behaviour clearly not aimed at
capturing or locating prey (e.g. perching, territorial behaviours or prey transport) was ignored (i.e., not quantified or
included in the analyses). First-year males or juveniles in
late summer can potentially be confused with females. If a
hunting bird was positively identified as belonging to either
group, it was excluded from the analyses, but observations
of ‘‘females’’ may involve a few cases of misidentified young
birds. The probability of this is however low: most first-year
males were observed at the beginning of the breeding season, when data on female hunting were not analysed, and
the data collection had finished in all sites before more than
a few chicks had fledged.
2.3.
Habitat data
For each of the three study sites, field data were used together
with Landsat 7 ETM images to characterise broad habitat
types. The satellite images were acquired from the global land
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km edge / km2
30
25
20
15
10
5
0
0
20
40
60
80
100
80
100
% heather
30
km edge / km2
cover facility (GLCF) website and were all from the same period (summer 2000). A cloud mask was created on each of the
images, in order to remove areas under cloud or cloud shadow
from the classification process. Multiple large stands of
homogenous vegetation were identified on the images and
verified on the ground for use as training data, and were digitised on-screen for each of the SPAs, using ArcGIS. Using
these as training sites to develop spectral signatures, supervised classification was carried out on the Landsat ETM+
images of each of the SPAs in Erdas Imagine (Leica GeoSystems). These images provided accurate information on land
cover distribution across the study sites (76% to 80% for the
mainland use types, using confusion matrices, Chapman
and Hughes, 2005). Habitat was only measured once in the
study, but moorland vegetation is not particularly dynamic,
but is slow to change. In the case of Langholm, visual comparison of satellite images from 1990 to 2000 does not appear to
show any substantial changes in the extents of the habitats
we are interested in between both periods, so our habitat data
are probably appropriate to use with Langholm data collected
in 1994–1999.
The choices of moorland vegetation classes were based on
the national vegetation classification (NVC), and condensed
to suit the purposes of this study. Stands of the most common
moorland vegetation types had their own class, and mosaics
and mixtures of vegetation types were also classified. Vegetation classes were decided as follows: dense heather: greater
than 70% heather cover; rough grass: greater than 70% rough
(unmanaged) grass cover; open heather (heather/grass mosaic): less than 70% but more than 30% heather cover; improved grassland: greater than 70% improved grass cover
(i.e., agriculturally improved, managed grasslands); woodland: greater than 70% wood cover; bracken: greater than
70% bracken cover; bog/mire: containing dominant (greater
than 50%) wet moorland species (e.g. cotton–grasses Eriophorum spp. and rushes Juncus spp., etc.).
We calculated the proportion of the main habitat types
(dense heather, open heather, rough grass, improved grass)
for each observation plot. Overall, heather (whether open or
dense) and rough grass comprised >70% of the habitat within observation plots, except for three in Galloway and 10 in
Orkney, where improved grass comprised >20% of the surface. When considering the three main habitats, observation plots included a wide range of values (Table 1). We
also calculated two further variables: total heather (as the
sum of open and dense), and heather to rough grass edge
density (hereafter heather edge, in km edge/km2 surface).
The latter was an indication of the landscape mosaic, and
was quadratically related to both rough grass and heather
(Fig. 2): highest values corresponded to areas where both
habitats were abundant and in equal proportion (i.e., surface covered roughly by 50% of each habitat). In moorland
areas managed for grouse shooting, another source of vegetation mosaic arises from regularly burning strips of
heather, to allow grouse access to heather of various ages
(Hudson, 1986). In our case, satellite images did not allow
to differentiate burned from unburned heather, so the
heather categories subsume all heather cover subjected to
rotational burning.
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25
20
15
10
5
0
0
20
40
60
% rough grass
Fig. 2 – Relationship between the percentage of heather or
rough grass cover, and the length of the edge between
heather and rough grass (each point represents an
observation plot).
2.4.
Statistical analysis
All statistical analyses were performed using SAS version 9
(SAS Institute). Our dependent variable was harrier hunting,
which was recorded in seconds. However, due to the highly
over-dispersed nature of the data, we grouped the amount
of observed hunting into minute groups; i.e., no hunting recorded = 0 min (this refers to either no birds observed during
the watch, or birds observed but involved in activities other
than hunting), 1–60 s = 1 min, 61–120 s = 2 min, etc. This allowed the data to be analysed using a Poisson error structure
with a log link function. We used a random effects model to
account for the lack of independence associated with repeated measures of observation plots in a given year, and of
the same observation plot between years. We thus used generalised linear mixed models with ‘‘observation plot’’, ‘‘year’’
and ‘‘observation plot*year’’ as random terms in the models.
For all analyses, because observation plots varied in size
and the watches varied in observation time (and it is more
likely to observe hunting in larger plots or in longer watches),
we incorporated the natural log of observation time (h) multiplied by plot size (km2) as an offset in the model. This effectively meant analyses were modelling hunting time (min
per h of observation) per km2. We subsequently call this variable ‘‘hunting intensity’’.
We included NPI in all models as a covariate. Additionally,
we included its quadratic term in initial models, as asymptotic relationships may be important (for example, if there
are plateauing effects). The proportions of the main
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vegetation types were intercorrelated (Table 1), meaning that
analyses combining the correlated variables were problematic. Therefore we included only one habitat variable at a time
in each model (in other words, we tested for the effect of each
habitat variable separately), and tested both linear and quadratic effects. Quadratic relationships with habitat are likely
where harriers favour mixed vegetation covers, and thus preference for a given habitat occurs at an intermediate level over
the range of that habitat.
For evaluating foraging patterns in males, we therefore
constructed six models (with the Glimmix procedure in
SAS). These included the random terms described above together with the following covariates or fixed effects: (i) NPI
and its quadratic term, (ii) one of the habitat variables (open
heather, dense heather, total heather, rough grass, improved
grass, heather edge density) and its quadratic term, (iii) breeding period (i.e., incubation vs nestling), hereafter ‘‘period’’, (iv)
study site (i.e., a three level factor), and (v) interactions of site
and period with NPI and the habitat variables, and the interaction between site and period. We then removed variables
(except NPI) sequentially (i.e., performing backwards selection, starting with the less significant variable), until the most
parsimonious model (including only variables with P < 0.10)
remained. Multi-model inference approaches could not be applied due to the inclusion of random effects in models. Analyses of female foraging patterns were only undertaken for the
nestling period, as they spend very little time hunting during
the incubation period. Thus, period (or any interaction with
period) was not included in any model; otherwise, analyses
were the same as for males.
To compare the explanatory value of each final model, we
calculated the proportion of explainable deviance explained,
comparing the covariance parameter estimates of the model
with just the random terms with those of the full model (Elston et al., 2001). The differences in the variance of the random effects between versions of the model provide
estimates of the variance due to each combination of fixed effects (Elston et al., 2001).
3.
Results
3.1.
Foraging patterns of male harriers
Hunting intensity by males varied between sites, probably
reflecting local harrier density (Table 2 and Fig. 3). Hunting
intensity by males also varied significantly with breeding period, being lower in the nestling than in the incubation period
(Table 2). Controlling for site and breeding period effects, all
models suggested that the effect of nest proximity is relatively
small, and that males hunt more in areas of mixed heather
and grass (Table 2 and Fig. 3): the quadratic relationship with
heather indicates that males avoided areas covered by more
than 50% heather; the relationship with open heather and
with heather edge density (in themselves, variables indicative
of heather–grass mosaic) was positive, and the latter also suggests an optimum heather–grass relationship of about 40–60%
(Fig. 3), equating to a heather edge density of ca. 16 km/km2,
(Fig. 2). Hunting intensity was lower in areas with larger proportions of improved grass cover at all sites (Fig. 3). There
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was a weak but positive linear relationship between hunting
intensity and rough grass cover, but the model including
rough grass was the poorest model in terms of explained deviance (Table 2). Overall, the models with best explanatory value
were those including improved grass and heather edge density, respectively (Table 2). Each of these two models explained
more than twice the deviance explained by site, period and
proximity to nest sites alone (Table 2).
The relationship between habitat and hunting intensity by
males did not change with breeding period in most models,
but there was a significant effect for the interaction between
(dense) heather and period (Table 2): the slope of the relationship was more marked in the incubation (0.051 ± 0.004) than
the nestling period (0.042 ± 0.014).
There were significant differences between sites in the
slope of relationships between foraging and open heather,
rough grass, heather edge density and improved grass (Table
2). However, examination of the actual relationships (Fig. 3,
Table 2) indicates that although the slope differed, the form
of the relationships remained constant, with either all positive or all negative relationships, except for rough grass. The
relationship for the latter variable was positive for two of
the study areas (Orkney and Galloway), but negative in Langholm, where rough grass was most abundant (Table 1).
3.2.
Foraging patterns of female harriers
Unlike males, inter-site differences in hunting intensity were
either non-significant or relatively small for females (Table 3).
The most important variable explaining female hunting was
proximity to nest sites (Table 3). The relationship between
hunting intensity and NPI was quadratic, with a maximum
hunting activity observed for NPI values between 5 and 10
(Fig. 4), which roughly corresponds to distances to the nest
between 300 m and 500 m. Hunting intensity was lower at
NPIs lower than 1.5–2 (i.e., areas further than 700–800 m from
the nest) or higher than 10 (i.e., areas within 300 m of nests).
Overall, female hunting patterns in relation to habitat
(after controlling for site and nest proximity) were similar to
those of males: hunting intensity was higher in areas of 30–
50% heather cover and lower outside this range (Table 3,
Fig. 4). Correspondingly, there was a significant relationship
between heather edge density and female foraging patterns,
which was (as for males) also quadratic. However, examination of the relationship (Fig. 4) suggests the optimum edge
density for females was only half that for males, indicating
different preferences in the intricacy of mosaic. The optimum
value varied significantly among the study areas (Table 3,
Fig. 4). There were no other significant differences between
sites in the relationships between female foraging and habitat
(Table 3). In contrast to males, the best model including habitat variables explained only 10% more than deviance explained by site and proximity to nest sites alone (Table 3).
4.
Discussion
Across three diverse study areas, patterns of hunting by male
and female hen harriers were similar with respect to habitat
selection but the effect of proximity to the nest site differed.
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Table 2 – Type III results of the six GLMMs models explaining male hunting in relation to period (incubation/nestling, or I/
N in Table), nest proximity index (NPI) and habitat variables. Initial models included Site (Langholm, Galloway, Orkney; or
L, G, O in Table), Period, NPI, a habitat variable (heather, open heather, dense heather, heather/grass edge density, rough
grass, improved grass), its quadratic term, and all double interactions. Parameter estimates for categorical variables
consider N or O as reference values.
Df
F value
P
Parameters
Explained deviance
Model 1
Intercept
Site (L/G/O)
Period (I/N)
NPI
Heather
Heather2
Heather*period (I/N)
NPI*period (I/N)
3.29
9.3
3.87
7.00
5.52
4.82
3.63
0.044
0.0001
0.049
0.010
0.022
0.028
0.057
1.567 ± 0.327
0.680 ± 0.293/0.044 ± 0.326
0.490 ± 0.162
0.11 ± 0.05
0.036 ± 0.015
0.0004 ± 0.0001
0.0063 ± 0.0029
0.105 ± 0.0549
23.5%
2
1
1
1
1
1
1
Model 2
Intercept
Site (L/G/O)
Period (I/N)
NPI
Dense heather
Dense heather2
D. heather*period (I/N)
NPI*period (I/N)
3.69
16.99
2.79
11.11
9.34
4.00
3.25
0.024
0.0001
0.095
0.0015
0.0032
0.046
0.072
1.551 ± 0.305
0.601 ± 0.295/0.237 ± 0.335
0.590 ± 0.143
0.098 ± 0.054
0.042 ± 0.014
0.0005 ± 0.0002
0.00087 ± 0.00043
0.0997 ± 0.0552
24.7%
2
1
1
1
1
1
1
Model 3
Intercept
Site (L/G/O)
Period (I/N)
NPI
Open heather
Open heather*site (L/G/O)
0.86
42.91
1.28
11.13
3.45
0.43
0.0001
0.26
0.0016
0.039
1.483 ± 0.288
0.512 ± 0.461/0.176 ± 0.289
0.587 ± 0.089
0.0156 ± 0.0138
0.119 ± 0.041
0.108 ± 0.042/ 0.087 ± 0.047
22.4%
2
1
1
1
2
1
2
0.02
34.54
4.91
4.59
4.63
3.18
0.94
0.0001
0.028
0.034
0.035
0.048
1.733 ± 0.402
0.158 ± 0.503/0.159 ± 0.739
0.549 ± 0.093
0.097 ± 0.044
0.0022 ± 0.0010
0.033 ± 0.015
0.037 ± 0.016/ 0.019 ± 0.019
19.1%
2
1
1
Model 5
Intercept
Site (L/G/O)
Period (I/N)
NPI
H–RG edge density
H–RG edge density2
H–RG E. dens*site (L/G/O)
1.65
45.05
0.72
8.44
4.51
4.59
0.19
0.0001
0.39
0.005
0.038
0.014
1.900 ± 0.347
0.177 ± 0.497/ 1.294 ± 0.73
0.601 ± 0.089
0.0113 ± 0.0134
0.232 ± 0.067
0.0074 ± 0.0035
0.091 ± 0.057/0.074 ± 0.068
30.6%
2
1
1
1
1
2
Model 6
Intercept
Site (L/G/O)
Period (I/N)
NPI
Improved grass
Improved grass*site (L/G/O)
6.69
45.43
0.26
10.26
4.52
0.0024
0.0001
0.60
0.0018
0.013
0.474 ± 0.228
0.910 ± 0.298/0.100 ± 0.34
0.601 ± 0.089
0.0067 ± 0.013
0.0148 ± 0.0055
0.174 ± 0.079/ 0.046 ± 0.022
35.8%
2
1
1
1
2
Model 7
Intercept
Site (L/G/O)
Period (I/N)
NPI
NPI2
4.42
34.99
4.13
3.17
0.016
0.0001
0.043
0.077
1.014 ± 0.221
0.720 ± 0.294/0.159 ± 0.336
0.548 ± 0.093
0.085 ± 0.041
0.0018 ± 0.0010
16.7%
2
1
1
1
Model 4
Intercept
Site (L/G/O)
Period (I/N)
NPI
NPI2
Rough grass
R. grass*site (L/G/O)
592
B I O L O G I C A L C O N S E RVAT I O N
0.6
Hunting (min/hr/km2)
Hunting (min/hr/km2)
0.7
1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6
0.6
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
80
90
100
0
10
20
40
50
60
70
80
90
100
80
90
100
2.5
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
1.5
1
0.5
0
0
5
10
15
20
0
25
10
20
heather-grass edge density (km/km2)
30
40
50
60
70
open heather cover (%)
0.8
Hunting (min/hr/km2)
2.5
Hunting (min/hr/km2)
30
total heather cover (%)
Hunting (min/hr/km2)
Hunting (min/hr/km2)
dense heather cover (%)
2
1.5
1
0.5
Galloway
0.7
Langholm
0.6
Orkney
0.5
0.4
0.3
0.2
0.1
0
0
0
10
20
30
40
50
60
70
80
90
100
rough grass cover (%)
0
20
40
60
80
100
improved grass cover (%)
Fig. 3 – Modelled frequency of male hunting in relation to habitat variables in the three sites. Values for the nestling period,
and nest proximity index (NPI) = 1. Graphs present values for the observed ranges of habitat values within each study site.
Both sexes favoured areas with heather–grass mosaics and
these patterns of habitat selection were general across the
study areas and throughout the breeding period. However, females exhibited this habitat selection mainly within 300–
500 m of nests whilst males ranged much more widely. Males
also avoided agriculturally improved grasslands, a habitat
rarely available to females foraging close to nests.
4.1.
Male and female foraging patterns
Our results on female foraging complement those of Thirgood
et al. (2003), who found that female hunting was positively
associated with NPI, heather cover and grouse abundance.
Their analyses, however, did not consider the relative importance of those three variables, which are potentially intercorrelated (Redpath et al., 1998; Pearce-Higgins and Grant, 2006).
Our study indicates that NPI has the strongest influence on
the amount of time an area was hunted by females. Hunting
was much less frequent both close to (<200 m) and far from
(>800 m) nest sites.
In contrast to females, the absence of strong relationships
between NPI and male hunting indicates that males are more
flexible in the areas in which they forage. These results agree
with others based on radio-tracked hen harriers, which
showed that females hunt primarily within 1 km from the
nest, whereas males hunt further away from the nests (up
to 3–4 km) and that there may be preferential directions in
the foraging trips of males (Arroyo et al., 2006). Our results
overall indicate that males hunt primarily in relation to habitat (and thus probably prey availability) within their home
ranges, irrespective of proximity to the nest site. Specifically,
males select areas of heather–grass mix but avoid areas of
agriculturally improved grass sward. This avoidance may be
explained by the fact that managed agricultural grasslands
support lower numbers of many important prey species than
either heather or unimproved grassland on one of the study
areas, Orkney, (Amar and Redpath, 2005), and the same relationships are to be expected elsewhere. Similarly, areas of
closed canopy woodland and bracken were avoided in western Scotland because prey there were scarce or difficult to
detect and capture (Madders, 2000).
Correspondingly, the selection by hunting male harriers of
areas of heather–grass mosaic accords with the fact that
areas with a high degree of mixture of heather and rough
grass contain high densities of both meadow pipits and voles
(Smith et al., 2001; Palmer, 2002; Vanhinsbergh and Chamberlain, 2001; Amar and Redpath, 2005; Pearce-Higgins and
Grant, 2006; Wheeler, 2008), important prey items for hen harriers (Redpath et al., 2002). Fragmented landscapes are also
associated with habitat heterogeneity, which may be associated with higher prey vulnerability (Sakai and Noon, 1997;
Bergman et al., 2006).
B I O L O G I C A L C O N S E RVAT I O N
593
1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6
Table 3 – Type III results of GLMM models explaining female hunting in relation to nest proximity index (NPI) and habitat
variables. Initial models included Site (Langholm, Galloway, Orkney; or L, G, O in Table), NPI, one habitat variable (heather,
open heather, dense heather, heather/grass edge density, rough grass, improved grass), its quadratic term, and all double
interactions. (When including improved grass, open heather or rough grass, only site, NPI and NPI2 were significant and
selected in the final model). Parameter estimates for categorical variables consider N or O as reference values.
Df
F value
P
Model 1
Intercept
NPI
NPI2
Heather
Heather2
10.87
4.06
5.21
5.64
0.0015
0.048
0.027
0.022
3.383 ± 0.432
0.869 ± 0.26
0.049 ± 0.024
0.049 ± 0.021
0.0005 ± 0.0002
26.3%
1
1
1
1
Model 2
Intercept
Site (L/G/O)
NPI
NPI2
Dense Heather
Dense Heather2
2.95
11.16
4.73
5.98
7.55
0.063
0.0013
0.033
0.019
0.0084
2.651 ± 0.425
1.063 ± 0.438/ 0.638 ± 0.509
0.906 ± 0.271
0.055 ± 0.025
0.050 ± 0.020
0.0007 ± 00002
30.3%
2
1
1
1
1
Model 3
Intercept
Site (L/G/O)
NPI
NPI2
H–RG edge density
H–RG edge density2
H–RG E. dens*Site (L/G/O)
4.59
10.58
4.17
16.75
15.44
4.31
0.011
0.0017
0.045
0.0001
0.0001
0.015
3.631 ± 0.561
1.232 ± 0.971/ 4.859 ± 1.644
0.801 ± 0.246
0.048 ± 0.023
0.528 ± 0.131
0.0314 ± 0.008
0.00749 ± 0.12/0.421 ± 0.147
37.1%
2
1
1
1
1
2
Model 4
Intercept
Site (L/G/O)
NPI
NPI2
2.88
18.37
8.37
0.068
0.0001
8.37
2.454 ± 0.352
0.952 ± 0.397/ 0.540 ± 0.462
1.055 ± 0.246
0.069 ± 0.023
27.3%
2
1
1
Habitat selection by females, after controlling for nest
proximity, was similar to that of males in that there was a
selection for hunting in areas with a heather mosaic. Females
hunted preferentially over heather, but avoided areas entirely
covered by dense heather, and they favoured areas where
heather edge density (and thus heather fragmentation) was
relatively high. However, the structure of the mosaic as measured by heather edge density indicates that females tended
to forage over less fragmented mosaics than males, for example where blocks of heather and grass were larger. This may
reflect the structure of the vegetation mosaics in areas holding the patches of mature heather cover favoured for nesting,
given that females tend to hunt mainly within a few hundred
metres of their nests.
While the preference for heather and rough grass cover
for foraging had been noted before (Thirgood et al., 2003;
Amar and Redpath, 2005), this study shows the simultaneous importance of both habitats, and the importance of
the habitat mosaic. Our results differ from one other Scottish study where harriers foraged preferentially in young
plantation forests (Madders, 2000, 2003), in association with
higher vole numbers and higher hunting success in that
habitat. Young forestry was not available in our study sites,
but Madders (2000) did suggest that the preference for young
forestry areas in his study may have been exacerbated by
very low prey availability in surrounding, heavily sheep-
Parameters
% Explained deviance
grazed moorland vegetation. There is thus scope for further
investigation of the relationships between land use, habitat
and prey abundance for better understanding hen harrier
foraging needs in areas where sporting and grazing management of moorland co-exist with areas of commercial upland
forestry. Similarly, it would be important to determine in the
future the optimal structure of the habitat mosaic (e.g. the
size of heather or grass patches that most benefits foraging
harriers).
4.2.
Implications for conservation management
Foraging habitat selection was mostly consistent across study
sites and throughout the breeding period. The only difference
arose for the relationship between rough grass and hunting,
which was positive for two of the study areas, but negative
in Langholm, the study area with a higher abundance of continuous patches of rough grass. Given that all other results
point at the preference of a mixture of heather and grass,
the negative relationship in this area may also concord to
the general result of a predilection for a habitat mosaic. Overall, this study shows the generality of these relationships
across a wide area of the species’ range in Scotland. Evidence
of such generality is increasingly recognised as being important when developing generic species management recommendations (e.g. Whittingham et al., 2003, 2007).
594
B I O L O G I C A L C O N S E RVAT I O N
Hunting (min/hr/km2)
4. 5
4
3. 5
3
2. 5
2
1. 5
1
0. 5
0
0
10
20
30
40
50
60
70
80
90
10
100
22
24
Dense
se heather cover (%)
Hunting (min/hr/km2)
5
4. 5
4
3. 5
3
2. 5
2
1. 5
1
0. 5
0
0
2
4
6
8
10
12
14
16
18
20
1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6
Possibly, the exact location of mosaics within breeding
areas may not be critical for males, which are the main food
providers during most of the breeding period (Redpath and
Thirgood, 1997; Leckie et al., 2008), at least if mosaics are
within 3–4 km of the nest. Because females hunt mainly
around the nests and also favour heather/grass mosaic areas
for foraging, mosaic areas may be of particular value to females when within a few hundred metres of the nest. Studies
have shown that having good foraging habitat close to nest
sites can affect breeding success (e.g. Tella et al., 1998; Rodriguez et al., 2006; Sergio et al., 2006; Amar et al., 2008; Hinam
and Clair, 2008).
Nests themselves tend to be located in patches of mature
heather cover. For example, Arroyo et al. (2006) found that
the probability of settlement to nest by female hen harriers
was highest when heather cover was 70–75% at the 1 km2
scale. A combination of heather–rough grass mosaic with
large patches of unfragmented mature heather cover may
therefore provide the optimum vegetation conditions for
hen harrier settlement and breeding. Again, it would be
important to determine the optimal structure and spatial
scale of this habitat mosaic.
Heather-grass
eather-grass edge density (km/km2)
4.3.
Implications for conflict management
Hunting (min/hr/km2)
6
Galloway
5
Langholm
4
Orkney
3
2
1
0
0
5
10
15
20
25
Nest Proximity Index
Fig. 4 – Modelled hunting frequency by females in relation to
nest proximity index and habitat variables in the three
study sites (for the observed ranges of values within each
study site) (graphs of habitat relationships assume nest
proximity index = 1).
On the whole, our results indicate that the creation or
maintenance of a heather/rough grass mixture and the avoidance of grassland improvement would be a beneficial conservation management strategy for hen harriers in most
moorland areas. One aspect to consider in terms of creating
this mosaic is that considerable heather loss has occurred
on moorland in recent years in many areas (Bennett, 1986;
Robertson et al., 2001), and further fragmentation of this
important habitat to create mosaic habitats for foraging harriers is likely to be undesirable (according to EU Habitats
Directive/UKBAP objectives for heather recovery). In contrast,
it would be beneficial for both upland heath and hen harrier
conservation to re-establish heather in areas where grassland
has wholly replaced heather cover, and to restore areas of improved (managed) grass to heather–grass mixes. Other work
suggests these mosaics are also likely to be beneficial for a
much wider spectrum of the moorland bird community
(Pearce-Higgins and Grant, 2006; Pearce-Higgins et al., 2008).
The findings from this study suggest that it may be possible
to apply habitat manipulation as a tool to reduce the predation impact of hen harriers in areas of conflict with grouse
management. Our results indicate that the peaks in male
and female harrier hunting occur at 40–60% heather cover.
Another study showed that red grouse densities in areas
with heather cover of 66–100% (a substantial proportion of
which were on driven grouse moor plots) are roughly double those at heather covers of 33–66% (Pearce-Higgins and
Grant, 2006). This suggests that, in principle, on grouse
moors where management is directed to maximise grouse
densities, there could be opportunities to ‘segregate’ the
areas holding highest grouse densities (i.e., areas with very
high heather cover) from those favoured for harrier foraging
(heather–grass mosaics). Furthermore, a previous study
showed that the proportion of heather around the nest
was associated positively with the number of grouse chicks
brought to the nest by females, but not by males (Amar
et al., 2004). Thus, if females settled in areas with less
heather around nests this would, in principle, reduce the
likelihood of females bringing grouse to the nest, whilst
having good foraging areas around them. A potential problem, however, is that harriers select heather-dominated
areas as nest sites (Arroyo et al., 2006). Therefore, it would
be critical to study how settlement may be influenced by
the availability of preferred foraging areas within a moor
before implementing this management technique.
Another area of human-wildlife conflict that has increased
rapidly in recent years is that between the conservation of
hen harriers and placement of wind turbines for renewable
energy generation (Madders and Whitfield, 2006). The results
from this study could be useful in reducing this conflict by
guiding the placement of turbines into areas that have the
lowest quality habitat for foraging harriers, or managing habitat within existing windfarms to reduce collision risk.
B I O L O G I C A L C O N S E RVAT I O N
In conclusion, this study showed that, while there is some
variation between areas in the factors that influence foraging
patterns of harriers, most relationships are general, and that
management recommendations at a wide geographical scale
are possible. It also confirms that habitat needs for foraging
may not be identical to those for nesting, as observed in other
species (Sergio et al., 2003; Carrete and Donazar, 2005), and
thus management of an area should take both requirements
into account. This study confirms the importance of considering foraging habitat preferences in the development of conservation and management programmes for protected
species, as these complement the management decisions
based solely on breeding habitat needs. The approach used
in this study could be also applicable to many other wideranging species.
Acknowledgements
Many people helped in the field over the years, and thanks are
due to Kerry Lock, Rory Gordon, Gaetan Bottin, Keith Fairclough, Lucy Bellini, Eric Meek, Brian Ribbands, Andy Knight,
Jim Williams and Stewart Williams. Chris Chapman and Rebecca Hughes created the habitat layer. We also thank Dave
Elston and James Pearce-Higgins for statistical advice, and
Des Thompson, Helen Riley, Adam Smith and James PearceHiggins for support and discussion. This study was a partnership project by Scottish Natural Heritage (SNH), Royal Society
for the Protection of Birds (RSPB), Game Conservancy Trust
(GCT) and Centre for Ecology and Hydrology (CEH), and was
funded by SNH.
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