Comparative nest habitat characteristics of sympatric White‐tailed Haliaeetus albicilla and Golden Eagles Aquila chrysaetos in western Scotland

Capsule Golden and White‐tailed Eagles selected different habitats for nesting.

Aim To investigate differences in nesting habitat used by sympatrically breeding eagles in western Scotland, following reintroduction of White‐tailed Eagles from 1975 onwards.

Methods Nest‐site locations from national surveys in 2003–05 were entered into a geographical information system (GIS) in order to measure a set of geographic parameters for each nest site. Binary logistic regression with backwards deletion of non‐significant terms was used to derive minimum adequate models at two spatial scales of the likelihood of an eagle nest belonging to one species or the other. We compared changes in occupancy between 1992 and 2003 of Golden Eagle territories inside and outside a GIS model of potential White‐tailed Eagle habitat and according to proximity to White‐tailed Eagle nests.

Results White‐tailed Eagles nested at lower altitudes than Golden Eagles, in more wooded habitats with more open water close by, tending to nest in trees where these were present. There were 3359 km² of potential White‐tailed Eagle nesting habitat within 25 km of existing White‐tailed Eagle nests, containing 54 Golden Eagle territory centres, but we found no difference in change of occupancy for Golden Eagle territories close to White‐tailed Eagles compared with those further away.

Conclusion White‐tailed and Golden Eagles appear to partition nesting habitat in the west of Scotland by altitude. This corresponds with behaviour in western Norway and with the situation described in historical accounts of nest‐sites in western Scotland prior to extinction of White‐tailed Eagles. It is also consistent with recent studies showing little overlap in breeding season diet of Golden and White‐tailed Eagles in western Scotland, and likely partitioning of foraging habitat by altitude. We conclude that the likelihood of competitive exclusion is less than previously suggested.     

The effect of extreme weather events on breeding parameters of the White Stork Ciconia ciconia

Capsule Although the White Stork avoids adverse weather conditions by modifying its arrival and breeding, it cannot avoid extreme weather events during the breeding season.

Aims To show how extreme weather conditions can influence breeding attempts of a large, long-lived species, the White Stork.

Methods We analysed data on arrivals of White Storks in Western Poland from 2005 to 2013 and detailed breeding biology parameters from 2009 to 2013 in relation to weather conditions. We analysed breeding success and breeding failure rate from 1974 to 2013.

Results In years with a cold March White Storks arrived later than when March was warmer. Frost during incubation negatively influenced the hatching success. Extreme weather events caused high late mortality even for nestlings older than 30 days. Data from 27 breeding seasons showed a significant increase in mean breeding success but also a significant increase in the proportion of pairs which lost broods in the nestling stage.

Conclusion The White Stork can modify its arrival in response to current weather conditions on the breeding grounds but it cannot respond to extreme weather events. Due to increasing frequency of extreme weather events caused by climate change, White Stork breeding success may decrease in the future.     

挠力河流域东方白鹳生境质量变化景观模拟

挠力河流域是濒危水禽东方白鹳主要繁殖区域。基于东方白鹳主要生境因子与景观植被类型之间的关系,利用GIS技术,以其生境类型图为基础,通过建立HSI模型,模拟评价了近40年来东方白鹳生境质量变化过程。结果显示（1）该流域湿地面积丧失了87％;（2）两种重要生境类型完全丧失,湖泊数量丧失93％左右,岛状林湿地数量丧失66％;（3）湿地景观的这些变化以及地理隔离导致了东方白鹳最佳适宜生境面积减少了95％,最小繁殖生境面积减少了97％;（4）东方白鹳种群数量迅速下降,20世纪80年代后其繁殖种群逐渐消失;（5）研究也显示,东方白鹳潜在的生境质量使该区依然具有恢复一定种群数量的能力。     

Breeding ecology and habitat selection of Merlin Falco columbarius in forested landscapes

Capsule: Long-term trends in Merlin Falco columbarius breeding performance remained stable during a period of extensive afforestation in Ireland, where Merlin predominantly select conifer plantations for nesting.

Aims: To determine breeding performance and habitat selection of Merlin in a landscape significantly altered by afforestation.

Methods: We compiled data on Merlin to determine long-term trends in breeding performance and to examine habitat selection in a country with one of the fastest rates of afforestation in Europe.

Results: Merlin predominantly nested in trees (99.5%; n?=?183 pairs), with a strong preference for conifer plantations, which accounted for over 12 times more nests than expected by random selection. Moors and heathland were strongly selected as land-uses adjacent to nest sites. Most nests were located within 10?m of the forest edge, and in forests aged between 31 and 40 years. Merlin showed positive selection for moors and heathland, peat bogs and natural grasslands within breeding territories, and breeding success was positively related to the proportion of these land-uses surrounding nests. Breeding was successful for 74% of nests (n?=?300), and mean productivity was 2.1 young per breeding attempt (n?=?265) between 1982 and 2014. Breeding parameters remained constant over the 33-year study period, despite an increase of more than 75% in forest cover during this time.

Conclusion: Merlin breeding performance showed no long-term effects of increased afforestation. Although Merlin predominantly nested in conifer plantations, the presence of nearby open suitable foraging habitats influenced nest site selection and breeding success. The nesting preference of Merlin makes them vulnerable to disturbance from forest operations, which requires mitigation.     

Effect of boundary vegetation and landscape features on diversity and abundance of breeding bird communities of abandoned crop fields in southwest Poland

Capsule The number of territories of birds nesting in abandoned crop fields was most strongly positively affected by field size and the presence of a railway embankment.

Aims To assess the influence of different habitat features on the number of birds nesting in abandoned crop fields.

Methods A breeding bird survey was conducted in spring 2002 using the mapping method on 67 abandoned fields located in southwest Poland. The associations between eight habitat features of the abandoned fields and the numbers of particular species and groups of birds, as well as the whole bird community, were tested with a glm.

Results The models for eight individual species revealed the following ranking, in terms of number of species affected by each predictor: area of fields (5), railway embankments and adjacent crop fields (4), roads and power lines (2), and number of trees (1). The number of the most numerous group of birds (species nesting on the ground, 79% of all breeding pairs) was affected by four predictors: field area, railway embankments, power lines and roads. The number of all birds was affected by six predictors: field area, railway embankments, power lines, roads, ditches and trees.

Conclusion Limited effects of trees, hedgerows and ditches on the number of territories suggest that abandoned fields without woody vegetation are attractive breeding habitats for many farmland birds. One of the effective ways of protecting farmland bird species in central Europe may be the preservation of abandoned fields in their present form, especially those <5 ha in area.     

Modelling the nesting-habitat of the Cinereous Vulture Aegypius monachus on a fine scale for conservation purposes

Capsule Nests of Cinereous Vultures were found to be located farther from roads, villages and the edge of large vegetation patches. They preferred large vegetation patches containing extensive Cork Oak cover on steeper slopes and with lower solar radiation. Less than 8% of the study area was predicted to be suitable for nesting.

Aims To generate a predictive habitat suitability map for the Cinereous Vulture's nesting-habitat on a fine scale for conservation applications within its breeding range.

Methods Habitat features of 43 nest-locations and random points were compared in order to identify nest-habitat selected in the region of the Hornachuelos Natural Park (Spain). A logistic regression approach was used to create habitat models.

Results Compared with random points, nests were found to be located farther from roads, villages and patch edges, and in large vegetation patches containing extensive Cork Oak cover on steeper slopes with lower solar radiation. The predictive map revealed that less than 8% of the study area had a greater probability of occupancy than 0.8.

Conclusions Most habitats in the study area are unsuitable for nesting suggesting that conservation of the best suitable areas is important. The fine-scale predictive map approach may be valuable in designating conservation priority areas.     

Between land abandonment and agricultural intensification: habitat preferences of Red-backed Shrikes Lanius collurio in low-intensity farming conditions

Capsule The conservation of Red-backed Shrikes on farmland habitats depends on extensive farming conditions.

Aims To evaluate breeding density and habitat preferences of the declining Red-backed Shrike and relate its occurrence to an environmental gradient ranging from land abandonment to intensive farming.

Methods The study was carried out in the Apennines (northern Italy), at nine 21-ha study plots. We identified factors affecting breeding density and habitat preferences at two spatial scales (landscape and territory) and analysed the variation in territory density according to the relative farming intensity in a low-intensity agricultural landscape.

Results The presence of shrubs and cultivated/grazed land positively influenced the number of Red-backed Shrike territories per plot, while the species' settlement within plots was related to higher values of shrub cover and the presence of hedges. Shrike occurrence was associated with land-use categories intermediate between land abandonment and intensive agriculture.

Conclusions This study provides a first detailed assessment of Red-backed Shrike habitat requirements in southern Europe. Favoured habitats were pasture/cultivation mosaics flanked by or interspersed with shrubs/hedges (15–20% of the surface of the 1-ha medium-sized territory). Thus, the conservation of Red-backed Shrikes in low-intensity southern European farmland appears to reflect a trade-off between agricultural intensification and long-term land abandonment.     

Predicting the distribution of a suitable habitat for the white stork in Southern Sweden: identifying priority areas for reintroduction and habitat restoration

The loss of wetlands and semi-natural grasslands throughout much of Europe has led to a historic decline of species associated with these habitats. The reinstatement of these habitats, however, requires spatially explicit predictions of the most suitable sites for restoration, to maximize the ecological benefit per unit effort. One species that demonstrates such declines is the white stork Ciconia ciconia , and the restoration of habitat for this flagship species is likely to benefit a suite of other wetland and grassland biota. Storks are also being reintroduced into southern Sweden and elsewhere, and the a priori identification of suitable sites for reintroduction will greatly improve the success of such programmes. Here a simple predictive habitat-use model was developed, where only a small but reliable presence-only dataset was available. The model is based on the extent and relative soil moisture of semi-natural pastures, the extent of wetlands and the extent of hayfields in southern Sweden. Here the model was used to predict the current extent of stork habitat that is suitable for successful breeding, and the extent of habitat that would become suitable with moderate habitat restoration. The habitat model identifies all 10 occupied nesting sites where breeding is currently successful. It also identifies ∼300 km² of habitat that is predicted to be suitable stork habitat, but that is presently unused; these sites were identified as potential areas for stork reintroduction. The model also identifies over 100 areas where moderate habitat restoration is predicted to have a disproportionate effect (relative to the restoration effort) on the area of suitable habitat for storks; these sites were identified as priorities for habitat restoration. By identifying areas for reintroduction and restoration, such habitat suitability models have the potential to maximize the effectiveness of such conservation programmes.     

Habitat associations and breeding success of the Chaffinch Fringilla coelebs

Capsule Population trends for Chaffinch on farmland are unlikely to be explained by their preference for non-crop habitats alone.

Aims To investigate the importance of non-cropped habitats for Chaffinch territory distribution, breeding success and foraging habitat selection in Scottish farmland.

Methods Territory distribution, nesting success and foraging behaviour of adults feeding chicks at the nest were recorded and related to habitat composition on two Scottish lowland farms.

Results Higher Chaffinch territory densities were associated with the presence of hedgerows, trees and grass leys, whereas lower densities were found adjacent to winter Barley. Nests were predominantly located in conifer trees and hedgerows. Trees were selected preferentially for foraging. Poor breeding success was detected in nests associated with cereal crops and wider field margins. Farmland in Scotland supported much lower territory densities than in England.

Conclusion Chaffinches rely predominantly on non-cropped habitats for territory establishment, nesting and foraging habitats. Differences in habitat composition between England and Scotland may explain differences in territory densities. However, the Chaffinch's preference for non-crop habitats is unlikely to explain its population trends alone. Immigration from other habitats and/or a decrease in inter-specific competition for resources may also have contributed to the increase in the national farmland Chaffinch population.     

Multi-scale habitat models for reintroduced bird populations: a case study of South Island saddlebacks on Motuara Island

Understanding resource selection by animals is important when considering habitat suitability at proposed release sites within threatened species recovery programmes. Multi-scale investigatory approaches are increasingly encouraged, as the patchy distribution of suitable habitats in fragmented landscapes often determines species presence and survival. Habitat models applied to a threatened New Zealand forest passerine, the South Island saddleback (Philesturnus carunculatus carunculatus), reintroduced to Ulva Island (Stewart Island) found that at landscape scale breeding pairs? preferences for sites near the coast were driven by micro-scale vegetation structure. We tested these results by examining models of breeding site selection by a reintroduced saddleback population on Motuara Island (Marlborough Sounds) at two scales: (1) micro-scale, for habitat characteristics that may drive breeding site selection, and (2) landscape scale, for variations in micro-scale habitat characteristics that may influence site colonisation in breeding pairs. Results indicated that birds on Motuara Island responded similarly to those on Ulva Island, i.e. birds primarily settled at the margins of coastal scrub and forest and later cohorts moved into larger stands of coastal forest where they established breeding territories. Plant species composition was also important in providing breeding saddleback pairs with adequate food supply and nesting support. However, Motuara Island birds differed in their partitioning of habitat use: preferred habitats were used for nesting while birds were foraging outside territorial boundaries or in shared sites. These differences may be explained because Motuara has a more homogeneous distribution of microscale habitats throughout the landscape and a highly bird-populated environment. These results show that resource distribution and abundance across the landscape needs to be accounted for in the modelling of density?bird?habitat relationships. In the search for future release sites, food (invertebrates and fruiting tree species) should be abundant close to available nesting sites, or evenly spread and available throughout the landscape.     

Conservation implications of past and present nesting habitat selection of the endangered Osprey Pandion haliaetus population of the Canary Islands

We studied nesting habitat selection of the endangered non‐migratory Osprey Pandion haliaetus population of the Canary Islands and evaluated the effect of human expansion in recent decades. Compared with randomly selected potential nest‐sites, Osprey nests were more frequently found on taller, southwest‐facing cliffs, characterized by lower human pressure and closer to Yellow‐legged Gull Larus michahellis colonies and Barbary Falcon Falco pelegrinoides breeding sites. Furthermore, changes in some breeding habitat features have been detected in recent decades. According to our predictive models, large areas of suitable habitat are available but unoccupied in the Canaries, and human activities are probably limiting the settlement and dispersion of new pairs.     

Agricultural intensification and the timing of breeding of Corn Buntings Miliaria calandra

Capsule In an intensively managed agricultural landscape, few females attempted a second brood.

Aims To investigate the effect of modern farming practices on the number of Corn Bunting breeding attempts.

Methods We compared the timing of breeding by Corn Buntings on the South Downs, West Sussex, with the habitat composition within 150 m of their nests.

Results Breeding was earlier in areas containing winter-sown wheat and set-aside than in areas containing spring-sown barley. Nests were earlier when cereal crops near the nest were more developed. The presence of unripe grain was a better predictor of the timing of breeding than the height of the crop. Double-brooding was extremely rare and few females re-laid after nest failures. Daily failure probabilities of clutches increased during the season, largely as a result of harvesting operations.

Conclusions Changes in the timing of cereal harvesting and the availability of uncultivated nesting habitat may have reduced the incidence of double-brooding in some intensive arable landscapes. Food availability may also limit the onset of breeding, further reducing the possibility of double-brooding.     

Willow Warbler Phylloscopus trochilus habitat in woods with different structure and management in southern England

Capsule Use of Light Detection and Ranging (LiDAR) data identified suitable Willow Warbler habitat based on mean vegetation height. This habitat model provided maps of distribution and occupation of suitable habitat.

Aims To identify habitat associations in woods with different vegetation structure and management systems during a period of low Willow Warbler populations.

Methods Locations of all Willow Warblers were mapped during the breeding season in three woods of contrasting management; recent low intervention, actively coppiced woodland and high forest with clear‐fells. Height profile models of each wood were derived from airborne LiDAR. The mean vegetation height at locations with Willow Warblers and a sample from the rest of the wood were used to produce models of optimum habitat and breadth of habitat occupied in each wood. The habitat model was then used to produce maps of suitable habitat.

Results The habitat models did not differ between woods, with highest probability of Willow Warbler occurrence in mean vegetation heights of 3.7–5.3 m. Habitat of heights 6–11 m appeared less suitable, being only partly occupied. Habitat maps showed that habitat of suitable height was only occupied when it occurred as large patches; smaller patches (mostly <0.5 ha) and edges along rides and fields were not used.

Conclusion The use of LiDAR derived measures of vegetation height identified areas of suitable habitat for Willow Warblers. Willow Warblers occupied areas of low mean vegetation height either as early successional or open canopy woodland in all woods. Height‐based habitat maps can identify areas of suitable habitat within larger expanses of heterogeneous woodland and are a potentially useful tool in assessing changes in extent of what are often temporary patches of habitat.     

Predictive habitat modelling to estimate petrel breeding colony sizes: Sooty shearwaters (Puffinus griseus) and mottled petrels (Pterodroma inexpectata) on Whenua Hou Island

Abstract

Between 2001 and 2006, we systematically sampled the entire coast of Whenua Hou, a rugged offshore island in southern New Zealand, to estimate the population densities of sooty shearwaters (Puffinus griseus) and mottled petrels (Pterodroma inexpectata) by counting the entrances of breeding burrows. A two‐step regression modelling process using binomial errors was used to predict the presence of a colony, and a normal general linear model was used to predict the density of entrances within colonies. Aerial photography, GIS and a Digital Elevation Model were used to extract relevant habitat and location variables, and a combination of both regression models was used to predict the density of breeding burrows within each 5.32 m² pixel on the island. This complex GIS and habitat prediction modelling approach gave population estimates very similar to a more traditional simple area extrapolation method and gave no improvement in precision. However, correction for the slope of the land increased our simple area estimates of population size by 11%. We estimate populations of sooty shearwater and mottled petrel breeding pairs at 173 000 (162 000–190 000) and 160 000 (123 000–197 000) respectively. Based on this number of breeding pairs, we calculate that Whenua Hou supports a total population of 868 000 (554 000–1 270 000) sooty shearwaters. Our estimate of the total mottled petrel population 202 000 pairs (162 000–242 000) is comparable with the only published estimate, but could be an underestimate because mottled petrels are sometimes found in large burrows. More research for robust estimation of population trends is needed to assess the conservation status of mottled petrels.     

Breeding dispersal in a heterogeneous landscape: the influence of habitat and nesting success in greater snow geese

Despite numerous studies on breeding dispersal, it is still unclear how habitat heterogeneity and previous nesting success interact to determine nest-site fidelity at various spatial scales. In this context, we investigated factors affecting breeding dispersal in greater snow geese (Anser caerulescens atlanticus), an Arctic breeding species nesting in two contrasting habitats (wetlands and mesic tundra) with variable pattern of snowmelt at the time of settlement in spring. From 1994 to 2005, we monitored the nesting success and breeding dispersal of individually marked females. We found that snow geese showed a moderate amount of nest-site fidelity and considerable individual variability in dispersal distance over consecutive nesting attempts. This variability can be partly accounted for by the annual timing of snowmelt. Despite this environmental constraint, habitat differences at the colony level consistently affected nesting success and settlement patterns. Females nesting in wetlands had higher nesting success than those nesting in mesic tundra. Moreover, geese responded adaptively to spatial heterogeneity by showing fidelity to their nesting habitat, independently of snowmelt pattern. From year to year, geese were more likely to move from mesic to high-quality wetland habitat, regardless of previous nesting success and without cost on their subsequent nesting performance. The unpredictability of snowmelt and the low cost of changing site apparently favour breeding-site dispersal although habitat quality promotes fidelity at the scale of habitat patches.     

Food from dumps increases the reproductive value of last laid eggs in the White Stork Ciconia ciconia

Capsule Accessing extra food from waste dumps increases egg volume and hatching mass in White Storks.

Aim To test how White Storks vary their investment in egg size, especially in last laid eggs, in relation to food availability, and to improve our understanding of the importance of extra feeding on intra-clutch variation.

Methods The study was carried out in three White Stork breeding colonies in northern Algeria. Breeding performance was recorded in 70 nests over three years. White Stork colonies situated close to chicken farms were considered to be part of a ‘pseudo experiment’ where parents had access to extra food. Egg volume, laying order, hatching order and hatching weight were recorded.

Results Egg volume and hatching mass in White Storks was significantly greater when they had access to extra food. The reproductive value of last laid eggs (fourth and fifth) doubled when females had access to extra food.

Conclusion Laying smaller last eggs within a clutch provides a mechanism to facilitate early brood reduction in the White Stork, and so should be advantageous when food is scarce. On the contrary, when females had access to extra food, last laid eggs were as big as first eggs which suggests egg size variation is adaptable to local conditions.     

Nest-site selection of the Long-eared Owl Asio otus in northwestern Switzerland

Capsules Sites are selected as part of an antipredator strategy.

Aims To assess if site choice depends on habitat variables at nest sites and if habitat quality influences reproduction.

Methods Breeding density was explored in northwestern Switzerland from 1992 to 1996. Habitat variables were examined at 38 breeding sites and were compared with data collected from random sites. Habitat quality was estimated using the discriminant function scores of the nesting sites.

Results Breeding density was found to vary between years; more pairs bred and raised more young in 1993, a year of high vole abundance. Long-eared Owls tended to avoid the vicinity of buildings; they occupied sites with denser forest edges, greater canopy cover, and with more conifers than random sites. I found no statistical evidence that they used less optimal sites when the population was high. The number of fledglings increased with habitat quality, but did not vary with any of the habitat variables taken separately.

Conclusion Long-eared Owl selects nesting habitat as part of an anti-predator strategy, but the measures of territory quality did not seem to be a limiting factor for the population.     

Long-term trends in the abundance of breeding Lapwing Vanellus vanellus in relation to land-use change on upland farmland in southern Scotland

Capsule A long-term decline on this habitat is shown to be associated with the intensification of agricultural management, particularly the occurrence of field drainage.

Aims To assess long-term trends in the number of breeding Lapwing and determine the relationship between these trends and changes in agricultural management on an upland study area.

Methods Breeding Lapwing were counted along two road transects in nine years between 1980 and 2002, and on one extensive plot in 1980, 1990 and 2000. Counts along the road transects were made from a vehicle and the fields used for nesting were recorded. Changes in field management along the transects were monitored annually between 1980 and 1990, and habitat composition assessed in 1980, 1985, 1990 and 2000.

Results During the first 20 years of study the number of breeding Lapwing declined substantially on all three count areas and by 77% overall, with further declines on both transects in 2002. The area of unimproved grassland and arable on these transects also declined substantially due to conversion to improved grass. Fields that comprised either unimproved grassland or arable were most likely to hold nesting Lapwing, while the chance of a field losing its nesting Lapwing was positively associated with the occurrence of drainage. Drainage and conversion to improved grass were closely linked.

Conclusions Agricultural intensification is a probable cause of decline in the number of breeding Lapwing in upland areas. Such declines may have been widespread in upland areas following increased agricultural intensification in recent decades.     

Week-end bias at observatories

Capsule Although subject to human disturbance Turtle Doves do nest successfully in these olive and orange orchards.

Aim To investigate the breeding ecology of Turtle Doves in a man‐made agricultural habitat in central Morocco.

Methods Turtle Dove nests were monitored in orange and olive orchards over three years (2006, 2007 and 2008). Nest abundance, nest location, egg‐laying chronology, clutch size, nest survival rates and breeding success were determined and compared between orchard types.

Results The Moroccan population of Turtle Doves start breeding earlier than European populations. Clutch size, nest survival rates and breeding success were similar in orange and olive orchards. Nest location differed between orange and olive trees. Nest densities were 16 nests/ha in olive orchards and 45 nests/ha in orange orchards. Nest success rate averaged 48%. Daily nest survival rates did not vary according to orchard types, year and date. In the two orchards, no nest position variables were significant predictors of nesting success.

Conclusion Although highly frequented by people, fruit orchards seem to be suitable breeding habitats for Turtle Doves in this region.     

The breeding status of Peregrine Falcons Falco peregrinus in the UK and Isle of Man in 2002

Capsule Numbers of breeding Peregrines are now higher than ever recorded and the recovery of the species within the UK has continued since 1991.

Aims To estimate the current Peregrine breeding population and its changes over time by recording breeding evidence and nesting range occupancy at known and potential Peregrine eyries.

Methods Attempts were made to check all potentially suitable Peregrine nesting locations for signs of Peregrine occupation. Causes of nest failure were documented where possible. Estimates of occupancy of nesting ranges that could not be visited were based on the proportion of ranges occupied from visited nesting ranges in the same region.

Results In 2002, 1530 Peregrine nesting ranges were estimated to be occupied, comprising 1456 pairs or single birds confirmed as in occupation, and 74 estimated extras (5% of total occupied nesting ranges) that were not visited. Including non‐breeding singletons holding nesting ranges, the UK and Isle of Man figure of ‘Ratcliffe pairs’ was 1437. This showed a 12% increase from 1991 and a 64% increase from the 1930s. However, there were geographical differences in change, with rapid expansion in the south of England tempered by declines in northern Scotland.

Conclusions The primary reasons for the population increase is primarily because of range expansion driven by a plentiful food supply and perhaps an increased tolerance of human disturbance. In areas where the species has declined, various factors (separately or in combination) including reduced food supply and persecution are likely to be involved.