Roost Choice Research Articles

The hidden cost of disturbance: Eurasian Oystercatchers (Haematopus ostralegus) avoid a disturbed roost site during the tourist season

Disturbance may impact individual birds and ultimately bird populations. If animals avoid disturbed sites this may prevent them from being disturbed directly but may also negatively impact their movement patterns and energy budgets. Avoidance is, however, challenging to study, because it requires following individuals over large spatial scales in order to compare their movement rates between sites in relation to spatiotemporal variation in disturbance intensity. We studied how 48 GPS‐tracked non‐breeding Eurasian Oystercatchers Haematopus ostralegus used two neighbouring roost sites in the Wadden Sea. One roost site is highly influenced by seasonal recreational disturbance whereas the other is an undisturbed sandbar. We analysed roost choice and the probability of moving away from the disturbed roost site with regard to a seasonal recreation activity index, weekends and night‐time. Oystercatchers often chose to roost on the undisturbed site, even if they were foraging closer to the disturbed roost. The probability that Oystercatchers chose to roost on the disturbed site was negatively correlated with the recreation activity index and was lowest in the tourist season (summer and early autumn), indicating that birds used the site less often when recreation levels were high. Furthermore, the probability that birds moved away from the disturbed site during high tide was positively correlated with the recreation activity index. The choice to roost on the undisturbed site implies that birds must fly an additional 8 km during one high‐tide period, which equates to 3.4% of daily energy expenditure of an average Oystercatcher. Our study tentatively suggests that the costs of avoidance may outweigh the energetic cost of direct flight responses and hence that avoidance of disturbed sites requires more attention in future disturbance impact studies. Nature managers should evaluate whether high‐quality undisturbed roosting sites are available near foraging sites, and in our case closing of a section of the disturbed site during high tides in the tourist season may mitigate much disturbance impact.

Mixed-species groups in bats: non-random roost associations and roost selection in neotropical understory bats

BackgroundMixed-species groups in animals have been shown to confer antipredator, foraging and other benefits to their members that may provide selective advantages. In most cases, however, it is unclear whether functional benefits are a principal driver of heterospecific groups, or whether groups simply result from simultaneous exploitation of common resources. Mixed-species groups that form independently of environmental conditions may, however, evidence direct benefits of species associations. Bats are among the most gregarious mammals, with sometimes thousands of individuals of various species roosting communally. Despite numerous potential functional benefits of such mixed-species roosting groups, interspecific attraction has never been shown. To explore alternative explanations for mixed-species roosting, we studied roost selection in a speciose neotropical understory bat community in lowland rainforest in Costa Rica. Long term roost data were recorded over 10 years in a total of 133 roosts comprising both natural roosts and structurally uniform artificial roosts. We modelled bat roost occupancy and abundance in each roost type and in forest and pasture habitats to quantify the effects of roost- and environmental variability.ResultsWe found that bat species presence in natural roosts is predictable from habitat and structural roost parameters, but that the presence and abundance of other bat species further modifies roost choice. One third of the 12 study species were found to actively associate with selected other bat species in roosts (e.g. Glossophaga commissarisi with Carollia sowelli). Other species did not engage in communal roosting, which in some cases indicates a role for negative interspecific interactions, such as roost competition.ConclusionsMixed-species roosting may provide thermoregulatory benefits, reduce intraspecific competition and promote interspecific information transfer, and hence some heterospecific associations may be selected for in bats. Overall, our study contributes to an improved understanding of the array of factors that shape diverse tropical bat communities and drive the dynamics of heterospecific grouping in mammals more generally.

Free-ranging bats alter thermoregulatory behavior in response to reproductive stage, roost type, and weather

Abstract Heterotherms vary their use of torpor and choice of refugia to deal with energetic stresses such as reproductive activity and extreme weather. We hypothesized that a temperate-region bat would vary its use of heterothermy in response to air temperature but use of torpor would also be influenced by reproductive stage and roost choice. To test this hypothesis, we collected data on skin temperatures of female Indiana bats (Myotis sodalis) carrying temperature-sensitive radiotransmitters during the summers of 2013–2015. We also measured internal temperatures and external characteristics of roosts used by these bats. We analyzed the influence of daytime air temperature, roost canopy closure, roost type, and bat reproductive stage, on daily heterothermy index and torpor characteristics of 17 bats during 103 full roost days (data collected consistently from when a bat entered its roost in the morning to when it emerged at night). Our data showed that Indiana bat heterothermy was influenced by reproductive stage, roost choice, and weather. Although they used torpor, pregnant bats were the least heterothermic (daily heterothermy index = 3.3 ± 0.6°C SE), followed by juvenile bats (5.6 ± 0.5°C), lactating bats (5.7 ± 0.5°C), and one postlactating bat (13.2 ± 1.6°C). Air temperature also influenced heterothermy of pregnant bats less than bats of other reproductive stages. Thermoregulatory strategies varied on a continuum from use of normothermia in warm roosts to use of long and deep bouts of torpor in cool roosts. The thermoregulatory strategy used seemed to be determined by potential reproductive costs of torpor and energetic consequences of weather. Because Indiana bats used different degrees of heterothermy throughout the summer maternity season, managers should offer maternity colonies an array of refugia to facilitate varying behaviors in response to weather and energetic demands.

Do bats use guano and urine stains to find new roosts? Tests with three group-living bats.

Many animals use social cues to find refuges. Bats can find roosts using the echolocation and social calls of conspecifics, but they might also use scent cues, a possibility which is less studied. The entrances of bat roosts are often marked by guano and urine, providing possible scent cues. We conducted eight experiments to test whether bats use the scent of guano and urine to find potential roosts. In field experiments, we tested if Molossus molossus (velvety free-tailed bats) in Panama and Eptesicus fuscus (big brown bats) in Ohio would investigate artificial roost boxes that were scented with guano and urine more often than a paired unscented control. We did not detect any difference in flights near the scented versus unscented roosts, and we detected only one entrance into any artificial roost (scented). In six captive experiments, we tested for the attraction of Desmodus rotundus (common vampire bats) and Molossus molossus to areas scented with guano and urine, under several conditions. Results were mixed, but overall suggested that the scent of guano and urine does not act as a strong lure for the tested bat species. We suggest that further tests of olfaction-based roost choice in bats should manipulate existing scent cues on familiar roosts.

Woodland fragments in urban landscapes are important bat areas: an example of the endangered Bechstein’s bat Myotis bechsteinii

Urban areas are increasing globally causing fragmentation and loss of habitats for many forest dwelling species. At the same time new habitats are created, which may be exploited by species with a broad ecological tolerance. Bats form the largest mammal group living in cities in Central Europe. Species diversity might be higher in some cities compared to rural areas. Yet, habitat-specialists struggle with rapid landscape change and urban development. The Bechstein’s bat Myotis bechsteinii is dependent on old growth deciduous forests that supply tree cavities to roost in. The use of small home ranges requires high prey abundance around roosts. Despite their habitat specialization we identified two Bechstein’s maternity colonies within the metropolitan area of Frankfurt/Main (Germany). The colonies numbered 25 and 31 adult females respectively. We radio tracked individuals from both colonies (n = 14) and compared their habitat use to that of females (n = 79) of 11 colonies from rural woodlands. We identified a total of 29 roosts used by the two maternity colonies during summer. Roost choice of the urban forest colonies was similar to the rural forest colonies with a preference for oak trees with woodpecker holes. The distances between roosts were similar between urban and rural colonies. Habitat selection was strongly towards old forest patches. We emphasize the importance of the persistence of old forest islands within urban areas to support remaining bat colonies with a network of bat areas, similar to the important bird areas (IBAs). Specialized species that fail to adapt to the rapid changes may survive within the urban environment relying on old forest patches.

No evidence for memory retention of a learned association between a cue and roost quality after hibernation in free‐ranging bats

AbstractHibernation is a physiological adaptation that allows animals to survive adverse environmental conditions. A commonly assumed cost of hibernation is impaired memory retention. So far, however, the effects of hibernation on memory retention have been assessed on only a few behavioral tasks, and exclusively under laboratory conditions. Taking advantage of the longevity and strict colony fidelity of female Bechstein's bats, we were able to evaluate memory retention in the same individuals over two consecutive summers in the field. We used a pairwise roost choice experiment with automatic monitoring of RFID‐tagged bats. Roosts’ suitability as day roost was associated with a distinctive external echo‐acoustic cue. Experiments were separated by a natural hibernation period of eight months. We determined associative learning and memory retention by comparing the bats’ proportion of visits to suitable roosts within and between breeding seasons, respectively. During the first breeding season, bats quickly learned to associate the suitable roosts with their external cue. After hibernating, we found no evidence that individuals remembered the association between the roosts’ suitability and their respective external cue, suggesting a lack of memory retention. Nevertheless, bats quickly re‐learned the same association during the second breeding season, emphasizing the high behavioral flexibility of Bechstein's bats.

Bat thermoregulation in the heat: Limits to evaporative cooling capacity in three southern African bats.

High environmental temperatures pose significant physiological challenges related to energy and water balance for small endotherms. Although there is a growing literature on the effect of high temperatures on birds, comparable data are scarcer for bats. Those data that do exist suggest that roost microsite may predict tolerance of high air temperatures. To examine this possibility further, we quantified the upper limits to heat tolerance and evaporative cooling capacity in three southern African bat species inhabiting the same hot environment but using different roost types (crevice, foliage or cave). We used flow-through respirometry and compared heat tolerance limits (highest air temperature (Ta) tolerated before the onset of severe hyperthermia), body temperature (Tb), evaporative water loss, metabolic rate, and maximum cooling capacity (i.e., evaporative heat loss/metabolic heat production). Heat tolerance limits for the two bats roosting in more exposed sites, Taphozous mauritianus (foliage-roosting) and Eptesicus hottentotus (crevice-roosting), were Ta=~44°C and those individuals defended maximum Tb between 41°C and 43°C. The heat tolerance limit for the bat roosting in a more buffered site, Rousettus aegyptiacus (cave-roosting), was Ta=~38°C with a corresponding Tb of ~38°C. These interspecific differences, together with a similar trend for higher evaporative cooling efficiency in species occupying warmer roost microsites, add further support to the notion that ecological factors like roost choice may have profound influences on physiological traits related to thermoregulation.

The avian "hibernation" enigma: thermoregulatory patterns and roost choice of the common poorwill.

Compared to mammals, there are relatively few studies examining heterothermy in birds. In 13 bird families known to contain heterothermic species, the common poorwill (Phalaenoptilus nuttallii) is the only species that ostensibly hibernates. We used temperature-sensitive radio-transmitters to collect roost and skin temperature (Tskin) data, and winter roost preferences for free-ranging poorwills in southern Arizona. Further, to determine the effect of passive rewarming on torpor bout duration and active rewarming (i.e., the use of metabolic heat to increase Tskin), we experimentally shaded seven birds during winter to prevent them from passively rewarming via solar radiation. Poorwills selected winter roosts that were open to the south or southwest, facilitating passive solar warming in the late afternoon. Shaded birds actively rewarmed following at least 3days of continuous torpor. Average torpor bout duration by shaded birds was 122h and ranged from 91 to 164h. Active rewarming by shaded birds occurred on significantly warmer days than those when poorwills remained torpid. One shaded bird remained inactive for 45days, during which it spontaneously rewarmed actively on eight occasions. Our findings show that during winter poorwills exhibit physiological patterns and active rewarming similar to hibernating mammals.

Stressful summers? Torpor expression differs between high- and low-latitude populations of bats

Variation in weather and food availability impacts the energy budgets of endotherms, with some species using torpor as an energy-saving strategy during periods of negative energy balance. We evaluated how latitudinal differences in energy balance relate to variation in torpor expression and roosting sociality between populations. We monitored summer skin temperatures (Tsk) of individuals from 2 populations of New Zealand lesser short-tailed bats (Mystacina tuberculata) separated by 6° of latitude using temperature telemetry. Although mean summer Ta was only < 1°C lower for the higher-latitude than lower-latitude population, individuals living at the higher-latitude site used torpor on 36% of observation days compared to 11% for lower-latitude bats. None of the recorded weather variables affected the propensity to enter torpor or torpor bout duration; however, the minimum torpid Tsk of bats positively correlated with daily minimum Ta. Roosts occupied by solitary bats were warmer than Ta, and temperatures within them fluctuated less than Ta. Higher-latitude individuals roosted solitarily (38%) more frequently than lower-latitude individuals (17%) and individuals from both populations exclusively used torpor while roosting solitarily. Arousals from torpor by higher-latitude bats coincided with sunset and not daily Ta maxima suggesting that bats were not fully exploiting advantages of passive rewarming. Site-specific roost choice and torpor patterns were apparent between M. tuberculata populations during summer, demonstrating that small differences in Ta differentially affect energetic strategy. The thermoregulatory behavior of species inhabiting latitudinal gradients in climate is highly plastic, likely to meet the specific challenges of their environment.

Winter climate affects torpor patterns and roost choice in New Zealand lesser short‐tailed bats

AbstractVariation in winter conditions differentially impacts thermoregulation of endotherms, with some species using torpor as an energy‐saving strategy during periods of negative energy balance. We evaluated how differences in energy balance due to winter ambient temperatures (Ta), would lead to differing torpor expression and roost choice between populations. Here, we monitored Ta and skin temperatures (Tsk) of New Zealand lesser short‐tailed bats (Mystacina tuberculata) using temperature telemetry from an inland population from Pureora, North Island and another from Hauturu/Little Barrier Island (Hauturu), an offshore island. Relative to Pureora, all recorded Ta variables were higher in Hauturu. Hauturu bats used short torpor (&lt;24 h) more frequently (51% of observational days) than their Pureora conspecifics (33%). Furthermore, minimum Tsk of Hauturu bats correlated positively with mean Ta and Hauturu bats were more likely to arouse on nights with warmer sunset Ta. No correlation was found for Pureora bats. Torpor bout duration did not differ between sites. On Hauturu, bats preferred thermally unstable roosts and preferentially roosted inside dead punga/silver fern‐trees (Cyathea dealbata). As punga thermal characteristics did not differ from Ta, Hauturu bats may take advantage of warm Ta to reduce rewarming costs. We did not observe differences in any thermoregulatory variables between sexes in Hauturu. Differences in winter Ta may differentially impact populations across a latitudinal gradient. Although bat populations in warmer climates are under less thermoregulatory stress, site and climate specific adaptations appear to be employed that affect roost choice and torpor patterns.

Cold and alone? Roost choice and season affect torpor patterns in lesser short-tailed bats.

Seasonal changes in weather and food availability differentially impact energy budgets of small mammals such as bats. While most thermal physiological research has focused on species that experience extreme seasonal temperature variations, knowledge is lacking from less variable temperate to subtropical climates. We quantified ambient temperature (T a) and skin temperature (T sk) responses by individuals from a population of New Zealand lesser short-tailed bats (Mystacina tuberculata) during summer and winter using temperature telemetry. During summer, communal roosts were more thermally stable than T a. During winter, solitary roosts were warmer than T a indicating significant thermal buffering. Communal roost trees were used on 83% of observation days during summer, and individuals occupying them rarely entered torpor. Solitary roosts were occupied on 93% of observation days during winter, and 100% of individuals occupying them used torpor. During summer and winter, bats employed torpor on 11 and 95% of observation days, respectively. Maximum torpor bout duration was 120.8h and winter torpor bout duration correlated negatively with mean T a. Torpor bout duration did not differ between sexes, although female minimum T sk was significantly lower than males. The summer Heterothermy Index varied, and was also significantly affected by T a. Mean arousal time was correlated with sunset time and arousals occurred most frequently on significantly warmer evenings, which are likely associated with an increased probability of foraging success. We provide the first evidence that torpor is used flexibly throughout the year by M. tuberculata, demonstrating that roost choice and season impact torpor patterns. Our results add to the growing knowledge that even small changes in seasonal climate can have large effects on the energy balance of small mammals.

Black or white? Physiological implications of roost colour and choice in a microbat

Although roost choice in bats has been studied previously, little is known about how opposing roost colours affect the expression of torpor quantitatively. We quantified roost selection and thermoregulation in a captive Australian insectivorous bat, Nyctophilus gouldi (n=12) in winter when roosting in black and white coloured boxes using temperature-telemetry. We quantified how roost choice influences torpor expression when food was provided ad libitum or restricted in bats housed together in an outdoor aviary exposed to natural fluctuations of ambient temperature. Black box temperatures averaged 5.1°C (maximum 7.5°C) warmer than white boxes at their maximum daytime temperature. Bats fed ad libitum chose black boxes on most nights (92.9%) and on 100% of nights when food-restricted. All bats used torpor on all study days. However, bats fed ad libitum and roosting in black boxes used shorter torpor and spent more time normothermic/active at night than food-restricted bats and bats roosting in white boxes. Bats roosting in black boxes also rewarmed passively more often and to a higher skin temperature than those in white boxes. Our study suggests that N. gouldi fed ad libitum select warmer roosts in order to passively rewarm to a higher skin temperature and thus save energy required for active midday rewarming as well as to maintain a normothermic body temperature for longer periods at night. This study shows that colour should be considered when deploying bat boxes; black boxes are preferable for those bats that use passive rewarming, even in winter when food availability is reduced.

Guided by the light: Roost choice and behaviour of urban Rainbow Lorikeets (Trichoglossus haematodus)

Abstract 1. The formation of large communal roosts is a conspicuous phenomenon associated with a wide range of bird species successfully exploiting urban environments. In many Australian cities, the abundance of the Rainbow Lorikeet (Trichoglossus haematodus), a native parrot, has increased markedly in recent decades, with the species roosting in very large numbers within suburban sites. These roosting locations are noisy and cause significant fouling of the land beneath, resulting in conflict with humans. 2. We investigated the selection of roosting sites in this species in Brisbane, Australia, by comparing characteristics of both the general sites of these roosts as well as individual trees used within roosting sites and trees that were avoided. 3. Lorikeets used a wide variety of tree types for roosting but demonstrated a clear preference for clumped trees within sparsely treed areas that received significantly more artificial light at night than otherwise suitable sites and trees nearby. 4. These features of roosting sites may enhance the detection of nocturnal predators by Rainbow Lorikeets, suggesting a potential positive impact of anthropogenic lighting. Our findings provide valuable insights into the management of roost-related conflicts in urban areas. We encourage further investigations into the possible benefits of artificial light.

Effect of roost choice on winter torpor patterns of a free-ranging insectivorous bat

Gould’s wattled bat (Chalinolobus gouldii) is one of only three native Australian mammals with an Australia-wide distribution. However, currently no data are available on the thermal physiology of free-ranging C. gouldii. Therefore, we aimed to quantify the effect of roost choice on daily skin temperature fluctuations during winter in C. gouldii living in an agricultural landscape in a temperate region. Ambient conditions consisted of long periods below 0°C and snow. Some individuals roosted high in dead branches whereas one individual roosted in a large cavity located low in a live tree. Torpor was employed on every day of the study period by all bats, with bouts lasting for over five days. The skin temperature of individuals in the dead branches tracked ambient temperature, with skin temperatures below 3°C on 67% of bat-days (lowest recorded –0.2°C). In contrast, the individual in the tree cavity maintained a larger skin-ambient temperature differential, likely influenced by the internal cavity temperature. Our study presents the lowest skin temperature recorded for a free-ranging Australian microbat and reveals that roost choice affects the thermal physiology of C. gouldii, ensuring survival during periods of cold weather and limited food supply.

Chemically-Mediated Roostmate Recognition and Roost Selection by Brazilian Free-Tailed Bats (Tadarida brasiliensis)

BackgroundThe Brazilian free-tailed bat (Tadarida brasiliensis) is an exceptionally social and gregarious species of chiropteran known to roost in assemblages that can number in the millions. Chemical recognition of roostmates within these assemblages has not been extensively studied despite the fact that an ability to chemically recognize individuals could play an important role in forming and stabilizing complex suites of social interactions.Methodology/Principal FindingsIndividual bats were given a choice between three roosting pouches: one permeated with the scent of a group of roostmates, one permeated with the scent of non-roostmates, and a clean control. Subjects rejected non-roostmate pouches with greater frequency than roostmate pouches or blank control pouches. Also, bats chose to roost in the roostmate scented pouches more often than the non-roostmate or control pouches.Conclusions/SignificanceWe demonstrated that T. brasiliensis has the ability to chemically recognize roostmates from non-roostmates and a preference for roosting in areas occupied by roostmates. It is important to investigate these behaviors because of their potential importance in colony dynamics and roost choice.

Roost availability may constrain shorebird distribution: Exploring the energetic costs of roosting and disturbance around a tropical bay

High tides force shorebirds from their intertidal feeding areas to refuges known as roosts. This paper explores the energetic costs of roost disturbance of great knot ( Calidris tenuirostris) and red knot ( C. canutus) at Roebuck Bay, North-western Australia, assessing disturbance levels at different roost sites through direct observation and automatic radio-telemetry, and applying physiological equations and predictive roost choice models to estimate energetic costs of disturbance through a complete tidal cycle. The study area had a variety of roosts, but use of each was constrained by conditions of tide and time. The roost most suitable for shorebirds on daytime high tides of intermediate height experienced high levels of disturbance from both natural sources (birds of prey) and humans. Flight costs caused by disturbance at this site exceeded the costs of flying to and roosting at the nearest alternative roost, 25 km away. However, shorebirds did not roost at the alternate site, possibly because of the risk of heat stress in a prolonged flight in tropical conditions. Increases in disturbance levels at just one of the roost sites of Roebuck Bay would increase energetic costs substantially, and could easily reach the point at which feeding areas accessed from this roost cannot be used without incurring a net energy deficit. Roost availability can therefore limit access to feeding areas and hence limit population size. Adequate provision and management of roost sites is accordingly an important consideration in conservation of sites used by coastal shorebirds.

High-tide habitat choice: insights from modelling roost selection by shorebirds around a tropical bay

High tides force shorebirds from intertidal feeding areas to sites known as roosts. We investigated the roost selection of great knots, Calidris tenuirostris, and red knots, Calidris canutus, on a tropical coastline in northwestern Australia, assessing several roost attributes and recording the frequency of use of each site through automatic radiotelemetry. To model roost choice we used two approaches: (1) conditional logistic regression models that assumed roost selection to be a trade-off based on a probabilistic assessment of several environmental characteristics; and (2) bounds-based models that assumed that birds selected the nearest roost site to their feeding grounds, provided that threshold values for certain environmental characteristics were met. Bounds-based models were more effective, and we suggest that they offer a closer approach to real roost choice mechanisms. By day, roost choice was affected by distance from the feeding area and microclimate; birds selected nearby roosts where they could stand on cool, wet substrates. Different roost selection criteria were used at night when birds chose safer, but more distant, roosts. Models that assumed that roost choice was influenced by recent experience of roost sites performed better than models that assumed constant assessment of roost quality. This effect was significant only at night, suggesting that memory was used more when information on roost quality was limited. Evidence that roost availability may influence selection of foraging areas is also presented. Our results suggest that shorebirds select roosts by using simple mechanisms, making roost choice models a potentially valuable tool in conservation planning.

Roost Structure, Modification, and Availability in the White‐throated Round‐eared Bat,Lophostoma silvicolum(Phyllostomidae) Living in Active Termite Nests1

ABSTRACTWe studied roost structure, modification, and availability inLophostoma silvicolum(Phyllostomidae), an insectivorous gleaning bat, on Barro Colorado Island (BCI), Panamá. Collection of nest material beneath termitaria and infrared video filming indicated that males ofL. silvicolumexcavate and maintain cavities inside active termite nests. A binary logistic regression analysis showed that to be suitable as roosts, termite nests have to be larger than 30 cm in diameter and taller than 30 cm, well shaded, with few transecting branches, and freely accessible from below. Use of active termite nests as roosts may provide several benefits toL. silvicolum, including reduction of competition for roost sites with sympatric bat species, reduced parasite load and a suitable microclimate. A comparison of number of all termite nests in selected forest plots with number of termite nests that are potentially suited as bat roosts and number of termite nests that are actually used by bats suggests thatL. silvicolummay not be roost‐limited on BCI in spite of its highly specialized roost choice.

High water roost selection by waders: maximizing feeding opportunities or avoiding predation?

During the highest spring tides the intertidal sediment flats of estuaries are fully inundated at high water, and waders have no choice but to move to supratidal roosts, e.g. on open farmland, saltpans or beaches. However, in many estuaries during the lowest neap or intermediate tides there are sectors of upper intertidal sediment flats that remain exposed even at the peak of high water, and so waders have the choice of roosting either there or in supratidal sites. In the Tagus Estuary, Portugal, as elsewhere, waders use both types of roosts during high water. Our main objective was to understand what makes waders opt for one of these two types of available roosts. We monitored wader use of saltpans and intertidal roosts from spring to neap tides, and measured foraging and alarm behaviour, prey availability and disturbance by predators. Most of the wader species studied chose intertidal (mudflat) roosts whenever these were available, and only roosted in saltpans during the peak of spring tides. We hypothesized that this preference was explained either by an attempt (i) to continue feeding into the high water period, or (ii) to minimize predation risk. Extending feeding time into the high water period did not seem to be very relevant for roost choice because both prey availability and foraging activity were low in both types of roosts. However, predator disturbance was several times higher in the saltpans than in the intertidal roosts, suggesting that this factor may be the determinant in the choice of roost type.

ROOST CAVITY SELECTION BY NYCTALUS NOCTULA AND N. LEISLERI (VESPERTILIONIDAE, CHIROPTERA) IN BIAŁOWIEŻA PRIMEVAL FOREST, EASTERN POLAND

za Primeval Forest in eastern Poland from May to August in 1998–2002. Because N. noctula is one of the most common forest-dwelling bats in Europe, whereas the smaller N. leisleri is relatively rare across its range (except in Ireland) and more limited to ancient forests, we hypothesized that these 2 taxa may differ significantly in their tree-cavity selection. Twenty-five N. noctula and 26 N. leisleri were radiotracked to 52 and 50 roost trees, respectively. For each accessible cavity roost occupied by N. noctula (n ¼ 28) and N. leisleri (n ¼ 39), 16 features were measured and compared with potentially available cavities (n ¼ 72). Both species were selective in roost choice, and preferred cavities located higher (averaging 19 m above the ground), in more open surroundings, with smaller entrances, and with greater safety distance (from martens) than available cavities. Nearly all roosts occupied by bats were dry inside. Both species slightly more frequently settled in cavities with entrances facing NE and SW, but the differences were not statistically significant. Compared to the pool of available cavities, N. noctula was statistically more frequently found in cavities with wider inside cross section and with 1 entrance, unlike N. leisleri, which often used cavities with more than 1 entrance (range 1–6 entrances). One of the most noticeable differences between the 2 species was roost origin. N. leisleri used natural cavities (90%) more often than woodpecker cavities (10%), whereas N. noctula showed the opposite tendency (woodpecker-made cavities accounted for just over half of roosts chosen by this species). The safety distance also was significantly larger in N. leisleri than in N. noctula. A logistic regression model for N. noctula incorporating 4 cavity variables (safety distance from martens, height above ground, cavity origin, and mean distance to nearest vegetation) classified roost and available cavities correctly 85% and 94% of the time, respectively. For N. leisleri, the use of 2 variables only (i.e., height above ground and marten distance) resulted in correct assignment of 85% of roosts and 88% of available cavities. These differences suggest that the 2 species use different antipredator strategies that may have important consequences for their different survival rates in younger forests. In general, roosts in Bia1owie_ za Primeval Forest are selected under pressures of predation and climate, and there bats tend to use safe and warm shelters.