Communal Roosts Research Articles

VEGETATIVE AND THERMAL ASPECTS OF ROOST-SITE SELECTION IN URBAN YELLOW-BILLED MAGPIES

We examined vegetative and thermal aspects of roost-site selection in urban Yellow-billed Magpies (Pica nuttalli) in Sacramento, California, from winter 2003 to spring 2004. Vegetation used for roosting included cultivated species such as glossy privet (Ligustrum lucidum), English ivy (Hedera helix), and white mulberry (Morus alba), and native species such as interior live oak (Quercus wislizeni), valley oak (Q. lobata), and California laurel (Umbellularia californica). Percent canopy cover was consistently high (mean = 94% ± 1.9 SD). Mean roost height was 9.7 m ± 3.5 SD and the mean height at which magpies roosted was 6.6 m ± 2.0 SD. Communal roosts were generally located within or near riparian corridors. Magpies roosted in relatively warm microhabitats, but they did not appear to obtain a thermal advantage by roosting communally. The timing of roost occupancy was restricted primarily to times when the roost was thermally advantageous.

Pair Roosting of Nesting Carolina Wrens (Thryothorus ludovicianus)

Abstract Carolina Wrens (Thryothorus ludovicianus), which maintain lifetime pair bonds and year-round territories, huddle in pair or communal roosts during the non-breeding season, particularly during cold winter nights. Pair roosting during the nesting season, however, is not known to occur. Here, we report huddled pair roosting by Carolina Wrens in Florida. The dates of pair roosting took place during nest construction through laying of the first egg (9–20 March 2004), and also on the date the fourth egg was laid in a clutch of five (24 March). The wrens roosted in a hanging flower basket located 2.4 m from their nest site. Although huddled pair roosting by wrens during periods of low ambient temperatures in the non-breeding season likely achieves thermal conservation, the benefits derived during the breeding season remain unclear. We discuss the possible thermoregulatory and pair-bond maintenance functions of pair roosting.

Thermoregulation under semi-natural conditions in speckled mousebirds: the role of communal roosting

Mousebirds (Coliiformes) exhibit well-developed communal roosting behaviour as well as a pronounced capacity for facultative hypothermic responses. We recorded body temperature (Tb)) in speckled mousebirds (Colius striatus) under semi-natural conditions in outdoor aviaries, and examined interactions between behavioural and metabolic thermoregulation by experimentally manipulating food availability and communal roosting behaviour. When food was available ad libitum, mousebirds roosting in a cluster maintained approximately constant rest-phase Tb, with 32°C < Tb < 42°C. By contrast, rest-phase Tb in single mousebirds decreased at 0.5°C/hr and minimum rest-phase Tb was significantly lower than when clustering. When food availability was restricted, the mousebirds exhibited facultative hypothermic responses that were less pronounced in clustering groups (minimum rest-phase Tb = 33.3°C, circadian amplitude of Tb = 9.5°C) compared to single birds (minimum rest-phase Tb = 30.7°C, circadian amplitude of Tb = 11.8°C). When clustering, rest-phase Tb was highly synchronized among individuals. Our data reveal that communal roosting has profound consequences for rest-phase thermoregulation in C. striatus, and provide further insights into the potential role of physiological constraints in the evolution of avian sociality.

Thermoregulation under semi-natural conditions in speckled mousebirds: the role of communal roosting

ABSTRACT Mousebirds (Coliiformes) exhibit well-developed communal roosting behaviour as well as a pronounced capacity for facultative hypothermic responses. We recorded body temperature (Tb) in speckled mousebirds (Colius striatus) under semi-natural conditions in outdoor aviaries, and examined interactions between behavioural and metabolic thermoregulation by experimentally manipulating food availability and communal roosting behaviour. When food was available ad libitum, mousebirds roosting in a cluster maintained approximately constant rest-phase Tb, with 32°C < Tb < 42°C. By contrast, rest-phase Tb in single mousebirds decreased at 0.5°C/hr and minimum rest-phase Tb was significantly lower than when clustering. When food availability was restricted, the mousebirds exhibited facultative hypothermic responses that were less pronounced in clustering groups (minimum rest-phase Tb = 33.3°C, circadian amplitude of Tb = 9.5°C) compared to single birds (minimum rest-phase Tb = 30.7°C, circadian amplitude of T...

Competing for position in the communal roosts of long-tailed tits

Communal roosting is widespread among social animals and has several hypothesized functions, including reduction of predation risk and thermoregulatory costs, and information sharing. The benefit derived from roosting in close proximity to conspecifics is likely to depend on an individual's position within the roost, but there have been few quantitative studies of the process of communal roost formation and the factors influencing relative positions. We studied the communal roosting behaviour of temporarily captive flocks of long-tailed tits, Aegithalos caudatus, a cooperative breeder that forms flocks during the nonbreeding season. Our objectives were to determine whether (1) individuals compete for particular positions within roosts, (2) individuals achieve consistent positions within roosts, and (3) an individual's roost position is a function of its dominance status. During roost formation birds were more likely to move to inner positions regardless of whether they started in an outer or inner position. However, as the number of birds in the roost increased during roost formation, birds in outer positions were less likely to relocate to an inner position. The same individuals occupied outer positions in a roost on successive nights, but there was no consistency in the occupation of specific inner positions. Birds occupying outer roost positions were significantly less dominant than those occupying inner positions. Our results show that long-tailed tits compete for inner positions within communal roosts, and that an individual's dominance status within a flock influences the outcome of this competition. (c) 2006 The Association for the Study of Animal Behaviour, Published by Elsevier Ltd. All rights reserved.

Minimal diurnal change in foraging time in an Australian passerine, the white‐browed babbler Pomatostomus superciliosus

This paper describes the daily foraging pattern of the white‐browed babbler Pomatostomus superciliosus, a small passerine endemic to Australia. The percentage of time spent foraging as the day progressed increased by an average of only 9%. This contrasts markedly with studies of other passerines that have mostly shown a bimodal pattern, with a peak of foraging early in the day followed by a period of reduced activity and a second peak later in the day. The most likely explanation for the observed pattern of foraging time is that babblers experienced considerable diurnal certainty in their food supply mainly as a result of stable and predictable weather conditions. The lack of a marked increase in foraging towards the end of the day might also have resulted from an increase in predation risk at this time and a reduced need to build up food reserves at the end of the day because of the energetic advantages of communal roosting in an enclosed nest.

Analyzing and modelling spatial distribution of summering lesser kestrel: The role of spatial autocorrelation

In modelling spatial distribution of species, ignoring spatial autocorrelation (SA) and multicollinearity may lead to false ecological conclusions. Here we take into account both issues for examining and modelling the spatial pattern of abundance of the globally threatened lesser kestrel ( Falco naumanni) during summer in a 38,400 ha area of northwestern Spain where large premigratory aggregations of the species occur. Spatial pattern was examined using Moran's correlogram, and models were built including geographical coordinates and autocovariate terms (which account for SA) in generalized linear models (GLM) and hierarchical partitioning (HP) models. HP models allow to alleviate multicollinearity. A grid-based approach was used by dividing the study area in 24 contiguous 4 km × 4 km squares where birds were counted in 2–3 visits per square (response variable). Environmental coarse-grained variables were extracted from a geographic information system (GIS) at three spatial extents. Moran's correlogram showed that lesser kestrel mean abundance per square was spatially autocorrelated up to 4–8 km. The results from both GLM and HP analyses were roughly compatible. The GLM models explained 80.0% of the variation in kestrel abundance and were the same at the three spatial extents. Lesser Kestrel abundance was not significantly explained by landscape variables, but was negatively related to both the distance to the nearest communal roost and distance to the nearest breeding colony with more of 10 breeding pairs of lesser kestrel. An autocovariate term added later in the GLM models improved both their explanatory power (from 74.5 to 80.0%) and model residuals, which were not longer spatially autocorrelated, fulfilling thus the statistical assumption of independent errors. Findings suggest that the spatial distribution of abundance of summering lesser kestrel is, at least, partially driven by endogenous causes, such as conspecific attraction. Exogenous causes such as finer-scale variables (e.g. type of crops and food available) are yet likely needed for lesser kestrel-environment relationships.

Group decision making in fission-fusion societies: evidence from two-field experiments in Bechstein's bats.

Group decisions are required when group coordination is beneficial, but individuals can choose between alternatives. Despite the increased interest in animal group decision making, there is a lack of experimental field studies that investigate how animals with conflicting information make group decisions. In particular, no field studies have considered the influence of fission-fusion behaviour (temporary splitting into subgroups) on group decisions. We studied group decision making in two wild Bechstein's bat colonies, which are fission-fusion societies of stable individual composition. Since they frequently switch communal roosts, colony members must regularly make group decisions over where to roost. In the two-field experiments, we provided marked individuals with conflicting information about the suitability of potential roosts. We investigated whether conflicting information led to group decisions that followed a 'unanimous' or a 'majority' rule, or increased colony fission. Individual behaviour suggests that bats considered both their own information and the behaviour of others when deciding where to roost. Group decisions about communal roosts reflected the information available to a majority of the bats roosting together, but conflicting information led to an increased fission in one colony. Our results suggest that fission-fusion societies allow individuals to avoid majority decisions that are not in their favour.

Effect of weather conditions on the communal roosting behaviour of common ravens Corvus corax with unlimited food resources

Avian time-activity budgets are known to be affected by many environmental factors. The aim of this study was to examine whether roosting times of common ravens Corvus corax are affected by inclement weather conditions in response to increased energy demands. The study was conducted at a communal roost of ravens in Thuringia, Germany, from August 2003 to January 2005. The number of roosting ravens fluctuated substantially, ranging from zero individuals in summer to 574 in winter. During the entire study period, a composting facility served as the main feeding ground for roosting ravens. There, the feeding-to-resting ratio did not depend on flock size, indicating the ravens were not food limited. Ravens arrived at the preroost and the roost later relative to sunset on cloudless evenings and on days with shorter length of daylight. Weather conditions measured as ambient temperature, wind speed, and cumulative daily precipitation had no effect on arrival times at the preroost and the roost. Likewise, moonlight did not affect roosting times. Apparently, flexible time-activity budgets resulting from a superabundant food supply may have enabled ravens to moderate the effects of environmental conditions on arrival times at the preroost and the roost.

Applying Network Analysis to the Conservation of Habitat Trees in Urban Environments: a Case Study from Brisbane, Australia

In Australia more than 300 vertebrates, including 43 insectivorous bat species, depend on hollows in habitat trees for shelter with many species using a network of multiple trees as roosts. We used roost-switching data on white-striped freetail bats (Tadarida australis; Microchiroptera: Molossidae) to construct a network representation of day roosts in suburban Brisbane, Australia. Bats were caught from a communal roost tree with a roosting group of several hundred individuals and released with transmitters. Each roost used by the bats represented a node in the network, and the movements of bats between roosts formed the links between nodes. Despite differences in gender and reproductive stages, the bats exhibited the same behavior throughout three radiotelemetry periods and over 500 bat days of radio tracking: each roosted in separate roosts, switched roosts very infrequently, and associated with other bats only at the communal roost. This network resembled a scale-free network in which the distribution of the number of links from each roost followed a power law. Despite being spread over a large geographic area (> 200 km2), each roost was connected to others by less than three links. One roost (the hub or communal roost) defined the architecture of the network because it had the most links. That the network showed scale-free properties has profound implications for the management of the habitat trees of this roosting group. Scale-free networks provide high tolerance against stochastic events such as random roost removals but are susceptible to the selective removal of hub nodes. Network analysis is a useful tool for understanding the structural organization of habitat tree usage and allows the informed judgment of the relative importance of individual trees and hence the derivation of appropriate management decisions. Conservation planners and managers should emphasize the differential importance of habitat trees and think of them as being analogous to vital service centers in human societies.

Roost site selection by lesser short‐tailed bats (Mystacina tuberculata) in mixed podocarp‐hardwood forest, Whenua Hou/Codfish Island, New Zealand

Roosting ecology of the lesser short‐tailed bats (Mystacina tuberculata) was studied in mixed podocarp‐hardwood forest on Whenua Hou/Codfish Island, New Zealand during winter 1996, and winter to summer 1998–99. The structural characteristics of roosts used by communal groups and by solitary bats are described, and roost selection is examined at three scales: forest stand, tree, and cavity. Differences in structural characteristics and microclimate of roosts used in winter and in summer are compared. Forty‐three bats were radio‐tracked and 122 tree roosts located. Communal groups occupied 47.5% of roosts and solitary bats occupied 52.5%. Bats selected roosts in Hall's totara (Podocarpus hallii) trees more than expected based on the availability of this tree species. Roost trees had significantly larger stem diameters and greater numbers of cavities than random trees. Communal bats demonstrated a higher degree of roost selectivity than solitary bats. Communal roosts had significantly less vegetation clutter surrounding roost entrances, larger stem diameters at cavity height, larger internal cavity heights, and thicker cavity walls than potentially available cavities. Communal roost cavities were consistently larger than solitary roosts. Solitary roosts had less open space around them than communal roosts and random cavities. Communal roosts were well insulated and exhibited stable internal temperatures. Structural and thermal characteristics did not differ between roosts used in winter and in summer. Differences between characteristics of communal and solitary roosts, and selection of well‐insulated cavities in some of the largest available trees, appears consistent among different forest types used by lesser short‐tailed bats in New Zealand, and with behaviour of many other tree‐dwelling bat species in temperate climates. Descriptive statistics of trees provide guidance to forest managers, especially those practicing silviculture, for identifying potential bat roosting habitat. Regression analyses of cavity characteristics provide insight into factors explaining the rarity of favoured cavities. Intraspecific differences in roost selection by bats should be considered when undertaking conservation management actions such as identifying trees for retention through logging cycles, or designating and designing reserves.

DIET INFLUENCES LIFE SPAN IN PARROTS (PSITTACIFORMES)

Abstract Evolutionary hypotheses regarding longevity predict that life span should increase as extrinsic mortality rates decrease. Specifically, a decline in age-specific survival and fertility should evolve and decrease life span in proportion to the magnitude of the mortality risk. We examined these ideas using a new data set on maximum longevity, ecology, and life history of 162 parrot species (Psittaciformes). Parrots are generally long-lived but exhibit remarkable variation in life span between similar-sized genera, with particularly long-lived species occurring in the Cacatua, Calyptorhynchus, and Amazona. After controlling for both body size and phylogenetic ancestry using a phylogenetic supertree of all 352 parrot species, type of diet and communal roosting explain significant variation in parrot life span, but the influence of communal roosting is statistically dependent on an association with diet type. We suggest that extreme longevity in parrots has evolved in response to species-specific char...

Diet Influences Life Span in Parrots (Psittaciformes)

AbstractEvolutionary hypotheses regarding longevity predict that life span should increase as extrinsic mortality rates decrease. Specifically, a decline in age-specific survival and fertility should evolve and decrease life span in proportion to the magnitude of the mortality risk. We examined these ideas using a new data set on maximum longevity, ecology, and life history of 162 parrot species (Psittaciformes). Parrots are generally long-lived but exhibit remarkable variation in life span between similar-sized genera, with particularly long-lived species occurring in the Cacatua, Calyptorhynchus, and Amazona. After controlling for both body size and phylogenetic ancestry using a phylogenetic supertree of all 352 parrot species, type of diet and communal roosting explain significant variation in parrot life span, but the influence of communal roosting is statistically dependent on an association with diet type. We suggest that extreme longevity in parrots has evolved in response to species-specific characteristics of diet, habitat, and behavior that influence extrinsic mortality rates.La Dieta Influencia la Longevidad en los Psittaciformes

Characteristics of diurnal roosts used by female Myotis bats in sub-boreal forests

Incorporating biodiversity objectives into industrial forest activities is a required part of sustainable management initiatives. In many temperate-forest environments, bats represent a large portion of the vertebrate species diversity, but for most locations we lack basic data regarding habitat needs. We describe characteristics of diurnal roost trees and stands used by radio-tagged bats in the sub-boreal forest of central British Columbia. We tracked 12 female Myotis lucifugus, 2 M. volans, and 1 M. septentrionalis to 33-day-roosts and found 4 additional roosts by random observations and inspecting cavities for guano. The 37 roosts occurred primarily in two species of trees, lodgepole pine ( Pinus contorta; n = 18) and trembling aspen ( Populus tremuloides; n = 17). In lodgepole pine, trees were in early stages of decay and bats roosted under bark, usually alone. Trembling aspen more typically contained groups of roosting bats (henceforth communal roosts; n = 14 trees). Communal roosts were all in vertical cracks that were formed from a combination of trauma causing a wood scar and decay fungi. Other species of trees used as roosts included black cottonwood ( P. balsamifera; n = 1) and hybrid white spruce ( Picea glauca x engelmannii; n = 1). We constructed logistic regression models for conifer and deciduous roosts used by M. lucifugus and M. volans to determine selection characteristics. Results from our analyses suggest that in sub-boreal forests, management of bat habitat should focus on the maintenance of: (1) mature stands; (2) individual lodgepole pine trees with exfoliating bark, few missing limbs, and free of potential obstructions to roost entrance; (3) large diameter trembling aspen, with long vertical cracks, in stands with fewer stems per hectare. No roosts were located in cavities excavated by woodpeckers, which implies that in our study area management plans that incorporate bats as secondary cavity users may not be sufficient to maintain populations.

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.

Use of the bait-marking method to estimate the territory size of scavenging birds: a case study on ravens Corvus corax

We tested the applicability of the bait-marking method in ravens Corvus corax to estimate their territory size during the breeding season in Białowieża Forest, eastern Poland. We marked 57 carrion baits and six additional carcasses with a distinctive type of plastic marker. The baits were experimentally placed around seven raven nests at different distances. A total of 1,018 pellets and 1,193 droppings from seven target raven nests, six neighbouring raven nests and from the communal roost were checked for markers. Ravens used 89% of baits and carcasses. In total, 705 markers, corresponding to 40 different baits and carcasses, were recovered. As confirmed by marker-type recoveries, ravens fed on 63% of experimental baits and 76% of those exposed inside the estimated territory. The proportion of baits utilised by a raven pair, as well as the total number of recovered markers declined with increasing distance of the bait from the nest. The probability of ravens feeding on a bait significantly decreased with greater distance from the nest. Distance from the nest correctly explained 84.5% of variation in the use of carrion by ravens. A threshold of 2,040 m indicated a raven territory size of 13.1 km2. Raven pairs utilised baits and carrion further away than 2,040 m only exceptionally. Habitat characteristics and snow cover did not affect bait use and marker recoveries. Bait-marking was revealed as an efficient, accurate and economic method to estimate the territory size of scavenging birds, as well as to collect information on their movements.

Nocturnal Breeding Behavior and Related Parental Investment of the Black Tern

Abstract Nocturnal incubating and brooding behavior of the Black Tern (Chlidonias niger) was studied at two colonies in successive years in The Netherlands. In late evening and early morning, nests were checked regularly for female or male nest attendance until or from complete darkness. During daytime females incubated eggs 60% of the time. At night, males took over completely, while females spent the night at a communal roost more than three km away from the breeding colonies. Few females stayed with their downy young at night and more females brooded young, as chicks became older. The total parental breeding investment (day and night) was equal (51% for males), but with higher nocturnal investment by males (79%). From six days after hatching, chicks were left unattended at night. The difference between male and female nocturnal investment might be explained by the preference for self-survival instead of clutch and brood survival. Males probably have greater interest in clutch survival than females, as ...

Gregarious foraging in barn swallows after the breeding season

Aerial vertebrate foragers, e.g. insectivorous bats, martins and swallows, often show gregarious behavior such as colonial breeding, communal roosting and aggregating behavior during foraging. Studies of gregariousness in aerial foragers have mostly focused on colonial breeding or communal roosting, and only a few intensive studies have dealt with gregariousness during foraging. Here, we report on large and stable aggregations of barn swallows, Hirundo rustica, observed during daytime after the breeding season in Japan. Relatively large aggregations of 20 or more birds were located around poultry, pig and cattle farms and mounds of manure. The aggregations were stable in size both within-days and between-days. Their activities consisted only of repeated cycles of foraging and resting around the farms where their prey, the large black soldier fly Hermetia illuceus (>10 mm) and other medium (5–10 mm) flies, was dense. Distributions of swallows around the farms overlapped with the distributions of prey, and the size of the aggregations significantly correlated with the abundance of prey.

Communal roosting in a suburban population of Torresian crows ( Corvus orru)

Communal roosting is a well-known phenomenon among numerous species of corvid (family Corvidae) worldwide. Surprisingly, in Australia this behaviour occurs only in certain urban populations of two species, and is unknown in any of the five native corvid species living away from urban areas. We studied the communal roosting behaviour of Torresian crows ( Corvus orru) in the suburbs of Brisbane, Queensland, Australia, by following six radio-tagged birds and by observing in detail the activities of crows as they departed and arrived at focal roosting trees. Several hundred birds were found to use the roosts on most nights, although there was clear seasonal variation; highest numbers were noted during the autumn and lowest were in spring. Radio-tagged adult birds tended to use the same roost regularly, while juveniles were far less faithful to a particular tree. Roosting behaviour was significantly influenced by cloud cover and temperature and the location of suitable pre-roost gathering sites. We were unable to determine why urban Torresian crows formed large roosting groups but our data did not support the information centre hypothesis.

Communal roosts as structured information centres in the raven, Corvus corax

Summary Ravens (Corvus corax, L.) feed on rich but ephemeral carcasses of large animals. Non‐breeding juveniles forage socially and aggregate in communal winter roosts, which may function as ‘information centres’ regarding food locations. In a large roost in North Wales, regurgitated pellets on the forest floor contained a variety of prey remains, which were more similar for ravens that had roosted close together the same night. Sheep carcasses placed at varying distances from the roost were baited with colour‐coded plastic beads. These were ingested and regurgitated in pellets back at the roost in aggregations, the spatial distribution of which consistently reflected the geographical location of bait sites. Aggregations of beads at the roost grew daily with an increasing radius centred upon the first pellet per carcass. This mirrored the linear increase of six birds per day in the size of groups flying between roost and carcass each morning. Rates of recruitment were greater for carcasses closer to the roost. Groups were led by a single bird roosting centrally within the aggregation. When individually identifiable (37·5% of cases), these individuals were dominant at the carcass and were among the minority of birds involved in acrobatic display flights at preroost gatherings. When contrasted with data on two alternative groups of ravens peripheral to the main roost which foraged and roosted collectively, these results provide strong circumstantial evidence for raven roosts as structured information centres. The adaptive basis for competitive recruitment resulting in excessively large group sizes is also discussed.