A comparative analysis of the diets of the Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus coexisting in the Alps

The diets of the Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus coexisting in the western Italian Alps have been compared by faecal analyses. A total of 1581 fresh droppings (405 of the Chough and 1176 of the Alpine Chough) were collected in the pastures of the Rhêmes Valley, Aosta, Italy, from June to November 1992 and analysed in the laboratory, and a mean volume percentage for each item was calculated. Both species were omnivorous, including animal, vegetable and mineral dietary fractions. There was no overlap in the vegetable part of the diet since the Chough fed virtually exclusively on Yellow Gagea Gagea fistulosa bulbs (dug out from the soil), which were not taken at all by the Alpine Chough. Conversely, the Alpine Chough fed on berries and hips from September to November, but these were virtually ignored by the Chough. Even though animal items were collected by both species, their preferences were different. In June, Alpine Choughs largely collected cranefly (Tipula) larvae whilst Choughs mainly fed on Lepidoptera larvae. From July onward, Alpine Choughs mainly consumed grasshoppers while Choughs also collected Tipula pupae, Lepidoptera and fly (Bibionidae) larvae and beetles (Scarabaeidae and Staphylinidae). Interspecific morphological and behavioural differences may be partly responsible for the segregation observed. Chemical composition and caloric contents of the food items suggest that the balance between costs of collecting and benefits of consuming may also contribute to diet differentiation. Both species took a broad spectrum of food, and there was no clear indication that the different population densities of the two Choughs in the Alps were directly correlated with diet, even though some data suggest that during autumn the Alpine Chough might have a diet better adapted to the high mountain environment than the Chough.     

Social structuring at a communal roost of Choughs Pyrrhocorax pyrrhocorax

The social structure of a communal roost of Choughs Pyrrhocorax pyrrhocorax was studied between July and September 1985. Different age classes within the roost were spatially segregated. Third-year birds were significantly more likely to attack another Chough and roosted in the densest part of the roosting flock. First-year birds roosted significantly lower down the roost cliff and on the periphery of the flock.     

The relationship between Red-Billed Chough Pyrrhocorax pyrrhocorax (L) breeding populations and grazing pressure on the Calf of Man

A link between the numbers of Loghtan Sheep Ovis aries var. and breeding pairs of Red-Billed Choughs Pyrrhocorax pyrrhocorax has been described for the Calf of Man, Isle of Man, UK, through the analysis of data from 1969 to 1982. Recent research has included further data on populations of Choughs and Sheep grazing, as well as data on brood sizes, Rabbit Oryctolagus cuniculus populations and summer rainfall. Data from 1969 to 1994 show a significant relationship between Sheep and Chough numbers, although data for the period 1983 to 1994 in isolation show no significant association. The mean (± se) brood size ringed in the nest in the period 1959–94 was 2.91 ± 0.011. Between 1972 and 1994, as Sheep numbers peaked, mean brood size decreased, only to rise again as Sheep numbers declined. High Rabbit populations correspond statistically with large Chough broods and increased fledging success. Declines in the Chough breeding population appear to be most closely linked to declines in Rabbit numbers due to outbreaks of myxomatosis, even during periods when Sheep numbers were maintained. The recovery of the Chough population after 1970 corresponds with a change in the effects of myxomatosis outbreaks. Rabbit grazing appears to be equally as important as Sheep grazing in the maintenance of the sward characteristics favoured by breeding Choughs. Combined data on Sheep and Rabbit grazing describe the fluctuations observed in Chough breeding success more fully than data for either grazer alone.     

A comparative analysis of the foraging behaviour of the Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus coexisting in the Alps

The foraging behaviour of the Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus was studied during summer and autumn in the western Italian Alps. We assessed feeding times, feeding rates and foraging techniques associated with different foraging habitats. The Alpine Chough is mainly a ground surface feeder, stays for a rela-tively short time at a feeding site (on average 2.1 min) and feeds quickly (on average 9.2 items/min). In contrast, the Chough is almost exclusively an undersurface feeder (digger and prober), stays at a feeding site twice as long as the Alpine Chough (5.4 min) and feeds four times as slowly (2.2 items/min). These differences suggest that the degree of actual ecological overlap is almost as low as possible for two species using the same Alpine pastures as foraging sites. Interspecific coexistence has occurred through a clear differentiation of foraging strategies and diets. The foraging efficiency (in terms of feeding rate)of the Alpine Chough was always higher than that of the Chough in all habitats where they occurred together. The Alpine Chough was more variable in the use of foraging techniques and more diversified in the use of foraging habitats than the Chough. In both species, juveniles fed less efficiently than adults; the foraging behaviour of the Chough is probably more difficult to learn than that of the Alpine Chough. Other data have also shown that the Alpine Chough is more opportunistic than the Chough in using seasonally available food. Considering the above, we suggest that the foraging behaviour of the Alpine Chough is more flexible and, perhaps, better adapted to the high mountain Alpine environment than that of the Chough. Two hypotheses concerning the ultimate reason why the Alpine Chough and the Chough have evolved divergent beak morphology are discussed.     

Seasonal variations in feeding range and flock structure of the Rook Corvus frugilegus in eastern Ireland

The feeding range and flock structure of Rooks showed temporal variations caused mainly by the dispersion of food and reproductive behaviour.
Feeding range was restricted in spring, autumn and early winter when food availability was high and Rooks were reproductively active. A large feeding range occurred in late summer and late winter, when food availability was either generally low or locally distributed and when Rooks were reproductively inactive. The occurrence of flocks common to several rookeries mirrored variations in feeding range; mixed rookery flocks were more common in late summer and late winter. Similarly, the size of the rookery (as measured by the number of nests) was related to feeding range only when Rooks were reproductively inactive and at such times larger rookeries had greater feeding ranges.
Flock structure showed similar seasonal variations; small widely spaced flocks predominated in summer, autumn, early winter and spring, whereas large dense flocks occurred in late winter. Diurnal variations in flock structure occurred within any one season. Both seasonal and diurnal variations in flock structure may be determined by the dispersion of the prey and the feeding strategy used to obtain it, reproductive behaviour and the risk of predation.     

Ecogeographic correlates of morphometric variation in the Red-billed Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus

We estimated the magnitude of intraspecific geographic variation in the Red-billed Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus from the analysis of external measurements of museum specimens collected throughout the Palearctic ranges of the species. By means of univariate and multivariate techniques we tested the importance of climatic and geographic factors as potential agents that might have shaped Red-billed and Alpine Chough morphology, discussing the dynamics of the interplay between natural selection, gene flow and phylogenetic constraints. Both species exhibited concordance in character differentiation. Bergmann's rule offered a valid explanation of Red-billed and Alpine Chough body size variation with the largest birds being found at higher elevation, or in colder and more arid regions. Shape was also important. The extremities of the body (bill and tarsus) were longer in warmer areas, in line with Allen's rule. In the Alpine Chough, temperature seemed to be the most important cause of body trait variation, whilst in the Red-billed Chough geographic distance among populations (a measure of geographic isolation) and altitude were also significant. In this paper we critically evaluate the validity of subspecific categories quoted in the literature, given that most variation appeared to be clinal.     

A comparative analysis of the breeding biology of the Red-billed Chough Pynhocorax pyrrhocorax and the Alpine Chough P. graculus coexisting in the Alps

We compare the breeding biology of a small population of Red-billed Chough Pyrrhocorax pynhocorax with that of the Alpine Chough P. graculus coexisting in the western Italian Alps. The analysis focused on nest site choice, timing of breeding, reproductive success and parental effort (assessed by means of behavioural indices such as food provisioning rates and time budgets inside and close to the nest). The Alpine Chough nested in a greater variety of sites: natural cliffs, windows of a dam, pot-holes and abandoned buildings; the Red-billed Chough used only natural cliffs. Laying dates also differed significantly, with the Red-billed Chough breeding one month earlier than its congener. Differences in the feeding ecology of those species might be responsible for the observed temporal partitioning, thus facilitating coexistence. Breeding success and reproductive behaviour were similar, and the species differed in only one out of eight breeding parameters. It is suggested that similar strong constraints on breeding behaviour have resulted in the similarities in behaviour seen in these species.     

Effects of nest-site availability and distribution on density-dependent clutch size and laying date in the Chough Pyrrhocorax pyrrhocorax

We compared the breeding phenology and clutch size of Choughs Pyrrhocorax pyrrhocorax in three Spanish areas that differed in feeding habitat, breeding environmental conditions and the distribution and availability of different nesting sites (cliffs or artefacts). The variation in the timing of laying seemed to be related to differences in climatic conditions determined by the altitude. The variation in clutch size was associated with the different use that Choughs made of the feeding areas because of the contrasting availability and distribution of their nesting sites. Within southeastern Madrid, clutch size of the Chough decreased and its variance increased at high nest densities on cliff sites, suggesting that density dependence in fecundity arises from habitat heterogeneity rather than by interference. At high densities, individuals, territories, nest sites or a combination of these may differ in quality, thus promoting differences and increasing the variance in the initial investment of reproductive effort. The syncronization of the onset of laying at increased breeding densities suggests that social influences arising from communal foraging or avoidance of predation by early warning and predator swamping may be acting also. We suggest that nest-site availability and distribution have a major influence on the social organization of Choughs through their breeding and foraging strategies.     

Linking territory quality and reproductive success in the Red-billed Chough Pyrrhocorax pyrrochorax: implications for conservation management of an endangered population

As changes in land use have been identified as the main factor explaining the decline in Red-billed Chough Pyrrhocorax pyrrochorax populations across western Europe, a study was carried out in Ouessant (western France) in order to assess the relationship between territory quality and reproductive success. As such an approach could be hindered by the fact that the birds' reproductive performance could be influenced by their breeding experience, we analysed both inter- and intranest-site variation in fledging success. Territory quality was quantified, combining habitat selection, territory size, the amount of feeding area and distance between nest and feeding area. Feeding habitats selected positively by Red-billed Choughs were characterized by a mean sward height of less than 5 cm. Foraging area was on average 21 557 m². Feeding areas (i.e. feeding habitat within foraging area) close to the nest were used preferentially. Fledging success appeared to be influenced neither by the total area of feeding sites in a Chough territory, nor by the mean feeding flight distance. However, fledging success adjusted to nest-site and year appeared to be influenced by feeding area close to the nest: one additional fledgling was expected for each additional 10 000 m² of feeding habitat within 300 m of the nest. These first results allowed us to consider recommending landscape management measures to ensure a favourable conservation status of local Chough populations.     

Vocalizations and morphology: interpreting the divergence among populations of Chough Pyrrhocorax pyrrhocorax and Alpine Chough P. graculus

Capsule Differences in vocalizations among populations are mostly explained on morphological bases, but historical factors may have played a significant role in differentiation processes.

Aims To investigate the relationships among vocal and morphological variation in two corvids: Chough and Alpine Chough.

Methods We used data from 11 populations of Chough and seven populations of Alpine Chough spanning the Palearctic distribution of the two species. Three data sets (morphometry, spectrotemporal parameters of trill calls and acoustic repertoire) were analysed and their variation compared with uni- and multivariate techniques.

Results In both species, morphological differences among populations were correlated to spectro-temporal variation of trills; in particular, frequencies of calls were negatively correlated to wing length (an indicator of body size). By considering only co-existing populations of the two species, the magnitude of morphological and spectrotemporal divergence was similar.

Conclusions In both species, birds from populations with similar morphology uttered similar call types and trills with close spectrotemporal features. In particular, larger-sized populations, as expressed by wing length, emitted lower pitched calls. However, the fact that dissimilarities in repertoire and morphology were correlated cannot be explained only by appealing to functional explanations, as the observed intraspecific morphological variability does not seem to be high enough to promote consistent changes in the pool of calls. In this case, historical factors might have contributed to the present pattern of differentiation.     

Diet of the Red-billed Chough Pyrrhocorax pyrrhocorax in south-east Spain

A sample of 140 Red-billed Chough pellets (35 from each season) was collected from communal roosting sites in south-east Spain. In almost every pellet there were three distinct dietary fractions: animal, vegetable and mineral. The animal faction constituted nearly 50% of the pellet volume in each of the four seasons. Wild grains and cultivated cereals were the most important vegetable elements, while 60% of animal prey (n= 3484) were beetles (mainly Tenebrionidae). There was marked seasonal variation in the composition of both the vegetable and animal fractions, the latter associated mainly with variation in the occurrence of Orthoptera, Lepidoptera larvae and Formicidae. Mixed flocks of Choughs and Jackdaws were common in the study area though agonistic interspecific interactions were never observed. The diets of the two species differ in that Choughs eat more beetles and wild seeds, whilst Jackdaws prefer ants and cereals. These observations support the hypothesis that competition from Jackdaws is not responsible for the reduction in the Chough's range.     

Effects of snow cover on the benthic fauna in a glacier-fed stream

1. Alpine streams above the tree line are covered by snow for 6–9 months a year. However, winter dynamics in these streams are poorly known. The annual patterns of macroinvertebrate assemblages were studied in a glacial stream in the Austrian Alps, providing information on conditions under the snow.
2. Snow cover influenced water temperature, the content of benthic organic matter and insect development. Taxa richness and abundance of macroinvertebrates did not show a pronounced seasonal pattern. The duration of the autumn period with stable stream beds was important in determining the abundance and composition of the winter fauna.
3. There were significant differences in species composition between summer and winter. Two potential strategies in larval survival were evident: adaptation to the extreme abiotic conditions in summer (e.g. Diamesa spp.) or avoidance of these conditions and development during winter (e.g. Ephemeroptera and Plecoptera).
4. A comparison of a stream reach with continuous snow cover and a stream reach that remained open throughout winter showed that conditions under snow are suboptimal. At the open stream site, with higher water temperatures and greater food supply (benthic organic matter content), abundance and taxa richness was higher and larval growth was faster. Several taxa were found exclusively at this site.
5. Winter conditions did not provide an entirely homogeneous environment, abiotic conditions changed rapidly, especially at the onset of snowfall and at snowmelt. Continuous monitoring is necessary to recognize spatial and temporal heterogeneity in winter environments and the fauna of alpine streams.     

Winter territoriality and flock cohesion in the black-capped chickadee Parus atricapillus

In northwest Massachusetts, black-capped chickadees shifted foraging groups repeatedly each day during the winter. As a result, there was no clear distinction between flock members and floaters, but instead a continuous gradation from birds that formed relatively stable associations to those that shifted groups more frequently. Group territories were absent and individual territories were at most poorly defended, if present at all. This picture differs from other recently studied populations, where flock membership is stable and flock territories are clear. Comparison with other populations indicates that winter social structure in the black-capped chickadee may be sensitive to both winter food levels and the proportion of year-round residents. This indicates that cost-benefit analyses of winter social organization should consider factors acting throughout the year as well as the residence status of the wintering birds. The data suggest that flock size, population density and home range size are sensitive to winter food on the study area, whereas territoriality and flock cohesion may respond more clearly to the local availability of breeding resources or the proportion of year-round residents.     

The ecological significance of extremely large flocks of birds

Population size is generally limited by resource availability during and outside the breeding season. Therefore, maximum size of flocks may provide important information on population regulation and the influence of diet and trophic level on maximal degree of sociality. We hypothesized that (a) flock size should increase with nutrient availability; (b) flock size should decrease with latitude because productivity is higher at lower latitude; (c) aquatic habitats should have larger flocks than terrestrial habitats because the former are less accessible; (d) smaller species should have larger flocks because they require overall less food; (e) human‐impacted species that live in perturbed habitats should have smaller flocks than other species; (f) flock size should decrease with increasing trophic level because there is a reduction in biomass due to conversion at each trophic level; and (g) flocks of species depending on ancestral landscapes should have decreased in size in recent years due to human impact (e.g., land‐use). We obtained 1564 observations of flocks that exceeded 100,000 individuals in order to test the predictions listed above. Most effect sizes were small to medium accounting for 1%–9% of the variance, while large effects accounting for 25% or more were only found for total nitrogen used per km² and area used for agriculture. Changes in large bird flocks were caused by habitat degradation and persecution, and temporal decline in size of large flocks revealed changes in nutrient use, reductions in nutrient cycling, and changes in flock size linked to trophic level.     

Spatial behaviour of adult male Alpine ibex<Emphasis Type="Italic">Capra ibex ibex</Emphasis> in the Gran Paradiso National Park,Italy

During a two year preliminary study, the spatial organization of a group of male Alpine ibexCapra ibex ibex Linnaeus, 1758 was examined in the Gran Paradiso National Park, Western Italian Alps, Italy. From December 1995 to January 1998 we measured annual, seasonal home range and home range during the rut, plus altitudinal migration of 13 radio-collared adult Alpine ibex. The small annual home range size showed a traditional use of space, confirmed by the high overlapping values between home ranges of consecutive years: the ibex used the same places from year to year. This was also true during periods of rut. Home ranges closely overlapped in consecutive ruts, while their size changed from winter to winter. Snow cover limited the movements of the ibex; winter and spring home ranges were smaller than those in summer and autumn. Mean vertical movement patterns were similar in the two years, showing the highest values in summer and the lowest in spring. Space use was never proportional to availability for each altitudinal range.     

The Effects of Predation Risk, Food Abundance, and Population Size on Group Size of Brown-Headed Cowbirds (Molothrus ater)

Social and ecological conditions can influence flock formation (e.g. number of flocks, flock size, etc.) depending on the degree of social attraction of a species. We studied group formation in brown‐headed cowbirds (Molothrus ater) over short time periods (30 min) in two semi‐natural experiments conducted under controlled conditions. First, we determined the shape of the relationship between intake rate and flock size by manipulating group size in a single enclosure. Second, we assessed the role of population size, food abundance, and predation risk, and their interactions, in flock size formation in a system of four enclosures (two with and two without food) connected to a central refuge patch. In the first experiment, we found that pecking rates peaked at intermediate flock sizes (three to six individuals), which was influenced by greater availability of foraging time and more aggressive interactions in large groups. In the second experiment, flock sizes in the patches with food increased with population size likely due to the benefits of patch exploitation in groups. Flock size decreased after predator attack probably because refuge availability reduced perceived predation risk more than flocking in larger groups. Food abundance had minor effects, varying flock sizes between the two patches with food, under high food availability conditions when population size was high, probably due to social cohesion effects. Our results suggest that: (1) this species has an inverted‐U food intake–group size relationship with a range of intake‐maximizing flock sizes rather than a single peak, (2) the presence of a near refuge modifies the expected benefits of group patch exploitation under high predation risk, and (3) an increase in population size would more likely be translated into rapid increases in the size of the flocks rather than in more new flocks.     

Influence of habitat and season on foraging flock composition in the European Starling (Sturnus vulgaris)

Summary This study examines the relative importance of habitat and season in determining size and composition of foraging flocks of European Starlings (Sturnus vulgaris). We hypothesized that if season was the principal determinant of flock size, variation across seasons (within habitat) would be greater than variation across habitats (within season). The calendar year was divided into 6 seasons corresponding to major stages in the annual cycle of starlings. After measuring the availibility of 15 foraging habitats occurring within the study area, we drove a 184 km route weekly (over a 17 month period) to census starling flock size, species composition, habitat use, and activity.Although flock size was influenced by both season and habitat, it varied relatively less within seasons (across habitats). Only five of nine seasons showed significant habitat effects, and two (of the five) were significant mainly because of large aggregations in habitats with unusually abundant food (e.g. feedlots, cornfields). In contrast, eight of nine habitats showed significant seasonal effects. The only exception (hay) was little used (<1% of all birds observed). For starlings the activities associated with seasonal events of the annual cycle were a major influence on patterns of social foraging. Habitat factors were less important because of constraints imposed by seasonal factors, but under some conditions (e.g. drought, clumped food supply) they became dominant.     

Costs and benefits of flocking in foraging white-fronted geese (Anser albifrons): effects of resource depletion

This study investigated the costs and benefits of flocking in white-fronted geese Anser albifrons foraging on rice grains in Japan. The time budgets of focal geese were recorded, and the effects of flock size on the proportions of time spent in vigilant and agonistic behaviour were tested. The results showed that the decline in vigilance level and consequent increase in foraging time were beneficial results of flocking whereas agonistic interactions, a potential cost of flocking, did not increase with increasing flock size. However, seasonal variation in flock size suggested that exploitative competition could be a cost of flocking; the sizes of flocks in spring, when resource depletion had progressed, were significantly reduced compared with those in autumn. An experimental increase in rice density resulted in a significant increase in flock size. We conclude that the flock size of foraging white-fronted geese is a result of compromise between a constant benefit of flocking (i.e. decline in vigilance level) and a cost of flocking varying with food abundance (i.e. exploitative competition).     

Lautinventar und Verst?ndigung bei Alpenkr?he (Pyrrhocorax pyrrhocorax), Alpendohle (Pyrrhocorax graculus) und deren Hybriden

Zusammenfassung Wir unterscheiden nach Zoo- (Alpenzoo Innsbruck, 32 Monate) und Freilandbeobachtungen im angeborenen Lautinventar von Alpenkrähe (AK) und Alpendohle (AD) 18 Rufe bzw. Lautgruppen. Hybriden verfügen über den doppelten Wortschatz, sind also zweisprachig. Die Überprüfung von Einzellauten in wiederholbaren Situationen ergab: Schon geringfügigen Verschiedenheiten kommt ein anderer Informationsgehalt zu. Bei AK und AD sind im Sonagramm, bei AD schon nach dem Gehör, zwei Lauttypen unterscheidbar, die wir mit Komm her! bzw. Geh weg! umschreiben. Sie entsprechen den Grundstimmungen der Vögel, dem Bedürfnis nach Kontakt bzw. Aggression. Variabilität des Stimminventars kann also bei diesen sozialen Felsenkrähen eine auch unserem Verständnis zugängliche Begriffs-Nuancierung bedeuten. Die interspezifische Verständigung geht so weit, daß sich auch im Freiland Mischpaare zusammenfinden können. Dann werden bestimmte Lautgruppen aus anderen Funktionskreisen verändert und für die Paarsynchronisation eingesetzt. AK und AD vermögen auch Artgenossen zu imitieren und in die Irre zu führen. Wir halten die verbale Kommunikation der beiden Felsenkrähen für ein weiteres Beispiel einer tierlichen Sprache.

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Vocal inventory and communication in the Chough (Pyrrhocorax pyrrhocorax), in the Alpine Chough (P. graculus) and in their hybrids

Summary The vocal inventory of the Chough (5 ), the Alpine Chough (5 ) and of 4 hybrids (P. p. × P. g.) as well as of their offsprings from two reproductive periods has been investigated for 32 months at Alpenzoo Innsbruck. There are some complementary observations in the field. In both species, 18 calls have been recognized, hybrids beeing bilingual. Minor differences in the calls may have a different meaning as has been proofed by recording calls and situations. Of special importance are two calls, which can be separated in the Alpine Chough already by ear, meaning Come! and Go away! with the basic motivations of need of contact and aggression respectively. Mixed pairs occuring in the field show, that there still is a possibility for interspecific understanding. Both Choughs may imitate members of their own species. They are artists in cheating too. In our opinion, verbal communication in these Choughs is in accordance with the definition of an animal language given byImmelmann.

     

The ecological significance of resting sites and the seasonal habitat change in polecats (Mustela putorius)

The daytime resting sites of 13 radio-tracked polecats ( Mustela putorius ) were recorded in a mountainous and a lowland study area in Switzerland. Information on breeding sites was collected from questionnaires distributed throughout the country. At all resting sites, the polecats were completely invisible from more than a metre distant. During summer, when polecats mainly live in forests, individuals used many different hiding-places (e.g. small self-dug burrows, woodpiles, heaps of branches and dry leaves, dense vegetation). Such places were used for short periods, and then abandoned. Above ground, the polecats sometimes built nests of dry grass or moss. In winter, the polecats slept mainly inside barns, stables and other buildings. These resting sites were changed less frequently. In rainy weather throughout the year, subterranean places were preferred. Breeding polecats were often found inside houses, and obviously did not avoid human presence.
The importance of different types of resting sites for polecats is discussed. In summer, the quality of a resting place is of less importance than its distance from the foraging area, but in winter, warm resting places are essential and buildings are therefore considered an important resource for polecats in Switzerland. The seasonal habitat change can more readily be explained as a consequence of thermoregulatory problems than of food availability, and the distribution of polecats in Switzerland may be affected by the availability of suitable winter resting places. The northern limit of polecat distribution and its historical changes can be explained by the changing availability of human buildings which provide winter resting sites.