Structure of a Brambling Fringilla montifringilla roost according to sex, age and body-mass

In a winter roost of several hundreds of thousands of Bramblings Fringilla montifringilla , the composition of birds roosting at the periphery and of birds flying into the centre of the roost was studied regarding sex, age, size and body-mass. At the periphery of the roost, more females were present than in the centre, and their proportion increased with decreasing density at the periphery. No significant difference in age classes was found between centre and periphery, but adults flew into the roost later than first year birds. Within sex-age classes, birds in the centre were heavier but not larger than birds at the periphery. The difference in body-mass between centre and periphery was more pronounced in small than in large birds within each sex-age class.
Because strong competition for particular positions within the roost was observed, it is inferred that the quality of roosting positions varied and that females and/or light birds were displaced toward the periphery. Predation and microclimate are discussed as possible reasons for the variation in the quality of roosting positions and for the lower energy reserves of birds roosting at the periphery.     

Roost site selection by Little Owls Athene noctua in relation to environmental conditions and life‐history stages

Roost site selection is a state‐dependent process, affected by the individual's costs and benefits of roosting at a specific site in the available environment. Costs and benefits of different roost sites vary in relation to intrinsic factors and environmental conditions. Thus, the cost–benefit functions of roost sites are expected to differ between seasons and life‐history stages, resulting in adjustments in roost site selection. Studying roost site selection throughout the year therefore provides information about year‐round habitat requirements at different life‐history stages. However, little is known about the roosting behaviour of birds. Here, the roost site selection of Little Owls Athene noctua was studied by repeated daytime location of 24 adult and 75 juvenile radiotagged individuals from July to November. Little Owls preferred sheltered roost sites such as tree cavities with multiple entrances. They increasingly used sheltered sites from summer to winter and preferentially used sheltered roost sites with low ambient temperatures. Juveniles used significantly less sheltered sites during dispersal than before and afterwards, and used less sheltered sites than adults within their home‐range. The survival probability of birds roosting frequently at exposed sites was reduced. Roost site selection is probably driven by the two mechanisms of predator avoidance and thermoregulation, and the costs of natal dispersal may include increased predation threat and higher energy expenditure for thermoregulation. We suggest that adequate roost sites, such as multi‐entrance tree cavities, are an important habitat requirement for Little Owls and that habitat quality can be affected by manipulating their availability.     

Experimental test of the mechanism underlying sexual segregation at communal roosts of harvestmen (Prionostemma spp.)

Closely related, ecologically similar species often roost in distinctly different habitats, and roosting patterns also vary within species in relation to sex, age and season. The causes of such variation are not well understood at either a proximate or ultimate level. We studied communal roosting in two congeneric species of Prionostemma harvestmen at a rainforest site in Nicaragua. Previous research showed that Prionostemma sp. 1 forms male‐biased communal roosts in tree‐root cavities, while Prionostemma sp. 2 forms communal roosts of variable but temporally stable sex ratios on spiny palms. Here, we investigate potential mechanisms underlying variation in roosting site choice between and within these syntopic species. First, we present the results of a field experiment designed to probe the mechanism underlying skewed roost sex ratios in Prionostemma sp. 2. Previous studies have suggested that these harvestmen use conspecific scent to locate communal roosts and that new roosts can be established via group translocation. Therefore, to test the hypothesis that skewed roost sex ratios in this species arise from sex differences in scent marks, we translocated single‐sex groups of ca. 30 individuals to each of 20 previously unoccupied spiny palms. Female release sites attracted new recruits of both sexes, while male release sites attracted almost exclusively males. We infer that Prionostemma sp. 2 females preferentially roost in sites scent‐marked by females and that this mechanism is sufficient to explain the skewed roost ratios. Further adding to knowledge of Prionostemma roosting behavior, we show that Prionostemma sp. 1 forms female‐biased communal roosts on spiny palms, that some roosts contain both species, and that the species composition is stable on a time scale of at least 2 weeks. To the best of our knowledge, this study is the first experimental test of mechanisms underlying sexual segregation at communal roosts in any taxon.     

Local density and group size interacts with age and sex to determine direction and rate of social dispersal in a polygynous mammal

Movement away from an area or social group in response to increasing density (density‐dependent dispersal) is known for most species; why it evolves is fundamental to our understanding of ecology and evolution. However, we have yet to fully appreciate how individuals of varying conditions (e.g., age and sex) might differently consider effects of density (quorum) when deciding to disperse or not, and scale dependence in their sense of quorum. We tracked movements of all individuals of a naturalized population of feral horses (Equus ferus caballus; Sable Island National Park Reserve, Nova Scotia, Canada) during a period of rapid population growth (N increased from 375 to 484 horses from 2008 to 2010). Permanent dispersal from breeding groups (bands) was positively density dependent for all age and sex categories with respect to local density (horses/km², bounded by the 99th percentile of individual movements [8000 m]), but was negatively and positively density dependent for males and females, respectively, in relation to group (band) size. Dispersal was generally female biased, with the exception of foals which moved with their mothers (no sex effect), and for yearlings and subadults when band sizes were smaller than average, in which case males dispersed at higher rates than females. Dispersal distance was positively related to local density. We conclude that dispersal rate can be both positively and negatively density dependent for feral horses, contingent on the state of individuals and the scale at which quorum with respect to choosing to disperse or not is assessed. Scale effects and interactions of density‐dependent and sex‐ and age‐biased dispersal may have both ecological and evolutionary consequences through effects on resource and mate competition.     

Marker-Based Relatedness Predicts Egg-Hatching Failure in Great Reed Warblers

I examine the relationship between reproductive success and marker-based relatedness (MBR; inferred from variation at 21 microsatellite loci) of pair-mates in a semi-isolated population of great reed warblers (Acrocephalus arundinaceus) over a 12 year period. There was a negative relationship between egg-hatching success and MBR, but no association between MBR and three other components of fitness (clutch size, proportion of fledglings and recruits). The relationship between hatchability and MBR was non-linear, with high hatching rates at low and intermediate levels of MBR and a sudden decline in hatching success among the small number of pairs with high MBR (<4% of the sample). It is likely that the decrease in fitness at high MBR was due to effects of homozygosity at genome-wide distributed loci, as opposed to effects of fitness loci in the local chromosomal vicinity of particular markers. This conclusion relies on the fact that similar results were found in another study of the same population that was based on DNA-fingerprinting band-sharing, and that there was a strong correlation between MBR and pedigree-based relatedness. The negative relationship between MBR and hatchability was especially pronounced in the first study years and levelled off in later years. This time-dependent effect of MBR on hatchability was not caused by a simultaneous temporal decrease in MBR, and may instead reflect the importance of other processes such as genotype × environment interactions or purging of deleterious recessive alleles.     

Ecological Correlates of Roost Fidelity in the Tent-Making Bat Artibeus watsoni

Roost switching is a common occurrence in bats, yet the causes and consequences of such behavior are poorly understood. In this study we explore the ecological correlates of roost fidelity in the tent‐making bat Artibeus watsoni, particularly focusing on the effect of sex, reproductive status, and roost availability using a three‐factor general linear model (GLM). We estimated roost fidelity of radio‐tracked individuals and found that the GLM was significant (R² = 0.72, F_10,34 = 8.91, p < 0.001). Significant interaction terms were observed for relative roost availability and sex (F_4,34 = 16.96, p < 0.001), and relative roost availability and reproductive status (F_6,34 = 7.62, p < 0.001), indicating that variation in roost fidelity among males and females, and among individuals under different breeding conditions, depended on relative roost availability at the site where they were radio‐tracked. Individuals in areas of high roost availability exhibited lower roost fidelity than those sampled in areas of lower roost availability. Females exhibited less roost fidelity than males for all roost availability categories, but the difference between males and females was only significant at high roost availability. The general pattern of decreased roost fidelity as roost availability increased was also prevalent among individuals in different breeding conditions. Additionally, satellite males exhibited higher roost fidelity than resident males in areas of low roost availability, and lactating females had higher roost fidelity than non‐breeding females in areas of medium roost availability. Our study thus demonstrates that sex, reproductive status, and roost availability all affect roost fidelity in the tent‐making bat A. watsoni, and also suggests that roost availability is the most important factor influencing roost fidelity in this bat, providing the first quantitative evidence that roost fidelity is correlated with roost abundance in a single species.     

Post-fledging recovery and dispersal of ringed Eurasian Sparrowhawks Accipiter nisus

Ring recoveries were used to explore the effect of early experience on the subsequent survival and dispersal of young Sparrowhawks in two areas in south Scotland. Young raised on high grade territories were recovered in greater proportion than young on low grade territories, implying that they had survived better after leaving the nest. Recovery rate tended to decline with increasing elevation of the nesting territory, implying that young from the high-ground territories survived less well than those from low-ground territories. Recovery rate showed no relationship with brood-size or sex composition, or with maternal age (yearling or older).
After the post-fledging period, dispersal distances were greater in females than in males and showed significant increases with increasing elevation of nesting territory (one area only) and with lateness of laying (or fledging) date. Regardless of laying date, young males from yearling mothers also dispersed further than young from older mothers, and young males from high elevation territories further than those from lower territories. No significant relationships emerged between dispersal distances and grade of natal territory, brood size or sex composition.     

Seed Dispersal of Morus macroura (Moraceae) by Two Frugivorous Bats in Xishuangbanna, SW China

The dispersal of Morus macroura seeds by two species of frugivorous bats ( Rousettus leschenaulti and Cynopterus sphinx ) was studied in a forest at Xishuangbanna Tropical Botanical Garden in Southwest China from March to May 2005. Feeding roosts were identified within 500 m around parent trees and the types and number of seed loads under each roost were recorded. We found feeding roost density decreased with increasing distance from the parent, but found no correlation between distance and seed deposition. The effect of bat digestion on seed germination was investigated, and we found that germination percentage of all treatments involving ingestion by bats was significantly lower than control seeds and some germination parameters of seeds from different treatments changed.     

Do hypotheses from short‐term studies hold in the long‐term? An empirical test

Abstract. 1. A sequence of population estimates for two now-extinct populations of Euphydryas editha bayensis is presented. After removing biased sampling days, estimates of demographic parameters from the long-term data were used to test five hypotheses built from studies of shorter duration. Such tests of short-term conclusions are rare.
2. The long-term demographic parameters include sex ratio, mortality, dispersal, mean flight date, and duration of flight season. The two populations differed with respect to sex ratio and mean flight date, and sexes differed with respect to mortality and dispersal.
3. Three of the five hypotheses were supported directly or indirectly by patterns in the parameters. These hypotheses predict that dynamics are asynchronous over the long term, that larval mortality, not adult abundance and mortality, is the primary determinant of changes in population size, and that topography mediates larval mortality.
4. Two hypotheses were not supported or supported only in part. Flight phenology differed between the study populations as predicted, but flight order was opposite that expected from the topographic composition of each habitat. Variability in sex ratio and the occurrence of female-biased ratios in the habitat of one of the populations also suggest that previous observations of sex ratio are not generalisable.
5. Populations were extremely volatile over the study period. Removal of biased sampling days did not change basic trends or fluctuations in the data. This volatility suggests that E. editha populations residing in similar habitats may risk immediate extinction.     

Flying-Fox Roost Disturbance and Hendra Virus Spillover Risk

Bats of the genus Pteropus (flying-foxes) are the natural host of Hendra virus (HeV) which periodically causes fatal disease in horses and humans in Australia. The increased urban presence of flying-foxes often provokes negative community sentiments because of reduced social amenity and concerns of HeV exposure risk, and has resulted in calls for the dispersal of urban flying-fox roosts. However, it has been hypothesised that disturbance of urban roosts may result in a stress-mediated increase in HeV infection in flying-foxes, and an increased spillover risk. We sought to examine the impact of roost modification and dispersal on HeV infection dynamics and cortisol concentration dynamics in flying-foxes. The data were analysed in generalised linear mixed models using restricted maximum likelihood (REML). The difference in mean HeV prevalence in samples collected before (4.9%), during (4.7%) and after (3.4%) roost disturbance was small and non-significant (P = 0.440). Similarly, the difference in mean urine specific gravity-corrected urinary cortisol concentrations was small and non-significant (before = 22.71 ng/mL, during = 27.17, after = 18.39) (P= 0.550). We did find an underlying association between cortisol concentration and season, and cortisol concentration and region, suggesting that other (plausibly biological or environmental) variables play a role in cortisol concentration dynamics. The effect of roost disturbance on cortisol concentration approached statistical significance for region, suggesting that the relationship is not fixed, and plausibly reflecting the nature and timing of disturbance. We also found a small positive statistical association between HeV excretion status and urinary cortisol concentration. Finally, we found that the level of flying-fox distress associated with roost disturbance reflected the nature and timing of the activity, highlighting the need for a ‘best practice’ approach to dispersal or roost modification activities. The findings usefully inform public discussion and policy development in relation to Hendra virus and flying-fox management.     

Dispersal of deer mice,Peromyscus maniculatus

Summary Dispersal of deer mice, Peromyscus maniculatus, was measured as immigration to and emigration from two control areas, and as immigration to a removal area. The number of mice dispersing was linearly related to the densities on the control areas, while the proportion of the population dispersing (rate of dispersal) was correlated primarily with the rate of increase of control populations. High rates of dispersal were also associated with a breakdown of the established social structure in the spring and fall. Dispersing animals were compared to residents with respect to sex ratio, weight, age, and breeding condition. The types of animals dispersing varied seasonally: light-weight, non-breeding males dispersed in the spring and summer; juveniles and breeding males dispersed at the end of the breeding season; and light-weight mice of both sexes dispersed over the winter. It is proposed that the animals that dominated the dispersal samples each season were moving in response to social pressure from residents, or local limitations of some resource, and thus, that dispersal was adaptive for the individuals concerned. Some tests of the hypotheses concerning resource limitation are suggested.     

Obligate cavity-roosting as a constraint on dispersal of green (red-billed) woodhoopoes: consequences for philopatry and the likelihood of inbreeding

Summary I studied and compared life histories and dispersal patterns of two populations of green (red-billed) woodhoopoes Phoeniculus purpureus, occupying different habitats (with widely different vegetation, topography and climate), over a period of eight years (258 flock years) in the eastern Cape Province, South Africa. The birds are obligate cavity-roosters, and I performed an experiment in which several woodhoopoe territories were established after the introduction of artificial roost sites in an area which previously supported no woodhoopoes. The evidence supports the hypothesis that roost cavities are critical in determining the limits to woodhoopoe distribution. Group size, dispersal frequency and dispersal distance differed significantly between the two study sites. I suggest that where cavities are in short supply and unevenly distributed, long-distance scouting forays are undertaken at a high probability of not finding a safe roost site to sleep in. This may result in an increased probability of predation, or physiological conditions with which an individual in poor body condition cannot cope. I propose that among woodhoopoes a stay-and-foray dispersal strategy is preferred to a depart-and-search strategy for a number of reasons. First, there is no within-group competition for limiting resources, since roost cavities do not provide a situation in which within-group conflict could arise. Second, I show that competition for breeding vacancies is more severe at the inland than at the coastal site, further borne out by the fact that, compared to coastal birds, inland ones are proportionally more likely to attain breeding status in the natal group than elsewhere. Finally, I propose that such environmentally-induced differential dispersal patterns between the two study sites have indirectly affected the frequency with which apparent inbreeding occurs, and I suggest that since incestuous pairings are relatively common, the either have no deleterious consequences for breeder fitness, or the potential costs of inbreeding are counterbalanced by the risks associated with dispersal.     

Dispersal of male ortolan buntings away from areas with low female density and a severely male-biased sex ratio

Dispersal is expected to enhance individual fitness, and individuals should thus disperse from areas with poor conditions to areas with more favourable conditions. Few studies have compared conditions before and after dispersal of the same individuals, and in birds little is known about the effects of sex ratio and female density on male dispersal decisions. In this study we examined various fitness-related parameters that adult male ortolan buntings, Emberiza hortulana, might use as cues in their decisions to disperse and settle. The study population has a strongly male-biased sex ratio. Using pairwise comparisons of pre- and post-dispersal conditions, we found that males moved from areas with low female density and a severely male-biased sex ratio to areas with higher female density and a less male-biased sex ratio. Male density and male age structure did not affect male dispersal and settlement. The sex ratio of the pre-dispersal sites was below the population average, but post-dispersal sites were not significantly better than the population average. This suggests that dispersal was triggered by poor conditions, whereas settlement may have been unrelated to the conditions at the new site. In the year of dispersal, males that undertook dispersal were less successful at acquiring a female than males that remained faithful to the site, but in subsequent years there was no difference. We suggest that dispersal might be adaptive under more natural conditions with a more balanced sex ratio. These results show that male ortolan buntings abandon areas with low female density and a severely male-biased sex ratio, and thereby end up in better places, although probably not better than the population average.     

Stable isotope ratios in winter‐grown feathers of Great Reed Warblers Acrocephalus arundinaceus,Clamorous Reed Warblers A. stentoreus and their hybrids in a sympatric breeding population in Kazakhstan

Analyses of the stable isotope composition of feathers can provide significant insight into the spatial structure of bird migration. We collected feathers from Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and a small sample of their hybrids in a sympatric breeding population in Kazakhstan to assess natural variation in stable isotope signatures and delineate wintering sites. The Great Reed Warbler is a long‐distance migrant that overwinters in sub‐Saharan Africa, whereas the Clamorous Reed Warbler performs a short‐distance migration to the Indian sub‐continent. Carbon (δ¹³C), nitrogen (δ¹⁵N) and deuterium (δD) isotope signatures were obtained from winter‐grown feathers of adult birds. There were highly significant differences in δD and less significant differences in δ¹³C between Great and Clamorous Reed Warblers. Thus, our results show that the stable isotope technique, and in particular the deuterium (δD) signal, resolves continental variation in winter distribution between these closely related Acrocephalus species with sympatric natal origin. The isotope signatures of hybrid Great × Clamorous Reed Warblers clustered with those of the Great Reed Warblers. Hence, a parsimonious suggestion is that the hybrids undergo moult in Afrotropical wintering grounds, as do the Great Reed Warblers. The observed δD values fell within the range of expected values based on available precipitation data collected at precipitation stations across the wintering continents of each species. However, the power to predict the winter origin of birds in our study system using these data was weak as the expected values ranged widely at this broad continental scale.     

Reed frog diversification in the Gulf of Guinea: Overseas dispersal,the progression rule,and in situ speciation

Oceanic islands accumulate endemic species when new colonists diverge from source populations or by in situ diversification of resident island endemics. The relative importance of dispersal versus in situ speciation in generating diversity on islands varies with a number of archipelago characteristics including island size, age, and remoteness. Here, we characterize interisland dispersal and in situ speciation in frogs endemic to the Gulf of Guinea islands. Using mitochondrial sequence and genome‐wide single‐nucleotide polymorphism data, we demonstrate that dispersal proceeded from the younger island (São Tomé) to the older island (Príncipe) indicating that for organisms that disperse overseas on rafts, dispersal between islands may be determined by ocean currents and not island age. We find that dispersal between the islands is not ongoing, resulting in genotypically distinct but phenotypically similar lineages on the two islands. Finally, we demonstrate that in situ diversification on São Tomé Island likely proceeded in allopatry due to the geographic separation of breeding sites, resulting in phenotypically distinct species. We find evidence of hybridization between the species where their ranges are sympatric and the hybrid zone coincides with a transition from agricultural land to primary forest, indicating that anthropogenic development may have facilitated secondary contact between previously allopatric species.     

Diurnal space use and nocturnal roost‐site selection by male Cerulean Warblers during the breeding season

Detailed information about space use during the breeding season is limited for most Nearctic‐Neotropical migratory species of songbirds because of their small size and often cryptic behaviors. We monitored male Cerulean Warblers (Setophaga cerulea), a species of conservation concern, using radio‐telemetry during the 2006–2008 breeding seasons in northern Alabama to better understand their space use and habitat selection. We estimated diurnal home range and core areas using information theoretic criteria, located nocturnal roost sites, and related day and evening locations to surrounding landscape habitat, including features representative of canopy disturbances. Mean home range size was 6.7 ha (N = 10), and home ranges included an average of at least 2 core areas encompassing 0.7 ha. We located 53 nocturnal roost sites that were an average 159.0 m from the center of the nearest core area. More than one‐third (36.6%) of roost sites were located outside the diurnal home ranges of male Cerulean Warblers; only 13.6% were located in core areas. Males in our study moved much farther than reported in previous studies, with some singing in areas > 300 m from previously used song perches, a behavior suggesting pursuit of extra‐pair copulations. Cerulean Warblers in our study preferentially selected a heavily forested landscape composed of mesic, floodplain bottomlands with little man‐made disturbance. Within their home ranges, diurnal locations of males in core areas were located significantly closer to a creek than locations outside of core areas. Our results suggest that male Cerulean Warblers require much larger areas than previously reported and underscore the importance of a predominately forested landscape in their habitat selection process. Although edge habitats appeared to influence space use by male Cerulean Warblers in our study, the extent to which this is an essential requirement is unclear. Our results and those of previous studies suggest that specific habitat requirements of this species can vary at the local scale throughout its breeding range.     

Coping with Shifting Nest Predation Refuges by European Reed Warblers Acrocephalus scirpaceus

Predation, the most important source of nest mortality in altricial birds, has been a subject of numerous studies during past decades. However, the temporal dynamics between changing predation pressures and parental responses remain poorly understood. We analysed characteristics of 524 nests of European reed warblers monitored during six consecutive breeding seasons in the same area, and found some support for the shifting nest predation refuge hypothesis. Nest site characteristics were correlated with nest fate, but a nest with the same nest-site attributes could be relatively safe in one season and vulnerable to predation in another. Thus nest predation refuges were ephemeral and there was no between-season consistency in nest predation patterns. Reed warblers that lost their first nests in a given season did not disperse farther for the subsequent reproductive attempt, compared to successful individuals, but they introduced more changes to their second nest sites. In subsequent nests, predation risk remained constant for birds that changed nest-site characteristics, but increased for those that did not. At the between-season temporal scale, individual birds did not perform better with age in terms of reducing nest predation risk. We conclude that the experience acquired in previous years may not be useful, given that nest predation refuges are not stable.     

Parasitization of bats by bat flies (Streblidae) in fragmented habitats

Parasites represent a large fraction of the world's biodiversity. They control host population sizes and contribute to ecosystem functioning. However, surveys on species diversity rarely include parasitic species. Bats often present traits favoring parasite diversity, such as large home ranges, long life spans, and large colonies. The most conspicuous bat parasites are the highly host-specific, blood-sucking bat flies (Diptera: Streblidae, Nycteribiidae). Recent studies have found a direct effect of habitat alteration on the abundance of bat species. We expected, therefore, that changes in the host community in response to anthropogenic habitat modification will also result in changes in the associated parasite community. We captured bats in three different habitats in Central Panama between 2013 and 2015. We recorded information on prevalence and intensity of bat fly parasitization of the seven most commonly captured bat species. Prevalence and intensity were both significantly influenced by roost type, abundance, and host sex and age. We found that habitat variables and matrix type significantly influenced the prevalence and intensity of parasitization, while the direction of the responses was host species- and parasite species-specific. In general, roosting conditions and behavior of host bats appear to be fundamental in explaining changes in prevalence and intensity of parasitization between different habitat types, as bat flies are bound to the roost during their reproductive cycle. Habitat alterations affect next to the host community composition also the availability of possible roost structures as well as microclimatic conditions, which all three reflect in parasitization.     

Differences in the ages, sexes and physical condition of Starlings Sturnus vulgaris at the centre and periphery of roosts

The differences between individuals at the centre and at the periphery of social groupings of birds have been demonstrated, but night-time roosts have received little attention. Here, the effects of age, sex and physical condition on the structure of Starling roosts were investigated. It was found that the dispersion of different age and sex categories was heterogeneous; proportionately more adult males occurred in the centre compared with the periphery, and proportionately more first-year females occurred on the periphery compared with the centre. Birds in the centre were heavier than peripheral ones irrespective of differences in body-size. We propose that the centre of the roost is the preferred location and that the observed dispersion reflects dominance.     

Microhabitat use and social structure in Linnet Carduelis cannabina and Corn Bunting Miliaria calandra at a winter roost in central Spain

Capsule Both species selected particular patches for roosting, and Linnets but not Corn Buntings showed a social structure with males occupying the preferred roosting positions.

Aims To evaluate microhabitat preferences of Linnet and Corn Bunting within a winter communal roost, and to examine roost structure according to sex, age and body size.

Methods We captured Linnets and Corn Buntings along a habitat gradient in a reed-bed in central Spain when they were coming to roost. Each bird was sexed, aged, and several body traits were measured. We also characterized the habitat patches where the birds were trapped and grouped patches according to their similarity.

Results Three groups of patches were defined according to reed structure. Linnets preferred patches with low density of high and thick shoots, while Corn Buntings preferably used patches with intermediate values for these variables. Interspecific competition for preferred positions was not evident. Male Linnets were proportionately more abundant in the preferred patches, and individuals in lower body condition, independent of sex, age or body size, occupied the non-preferred patches. Male Corn Buntings were less abundant, bigger in size, and showed better condition than females, although differences in body size and condition were not apparent among patches within the roost.

Conclusion Our results suggest that among-patch differences in structure and location within a reed-bed generate microhabitats that differ in quality for roosting passerines. This habitat heterogeneity in turn seems to be responsible for the social spatial structure observed in Linnets, in which females and individuals in poor physiological condition were displaced, possibly through competition, towards non-preferred positions. The relatively small body size of Linnets compared to Corn Buntings might explain Linnets' preference for patches that seem to provide more protection against harsh weather conditions.