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B I O L O G I C A L C O N S E RVAT I O N 1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/biocon 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). 1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6 587 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. 588 B I O L O G I C A L C O N S E RVAT I O N 1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6 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 B I O L O G I C A L C O N S E RVAT I O N 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. 589 1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6 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 590 B I O L O G I C A L C O N S E RVAT I O N 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 1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6 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. B I O L O G I C A L C O N S E RVAT I O N 591 1 4 2 ( 2 0 0 9 ) 5 8 6 –5 9 6 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. 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