A free-ranging maternity colony of big brown bats Eptesicus fuscus roosting in rock crevices along the South Saskatchewan River in south-eastern Alberta, Canada, was studied to understand better the discrepancy that exists in the literature regarding torpor use by reproductive female bats. Using radio-telemetry, thermoregulatory patterns and roost microclimate were recorded for pregnant, lactating and post-lactating females. Relative torpor use is described in several ways: the proportion of days on which torpor was used, depth, minimum body temperature, time spent in torpor, and a comprehensive torpor unit (degree-min). Pregnant and lactating female E. fuscus used torpor to the same extent overall (degree-min), but pregnant bats used torpor less frequently and with more time in deep torpor. Torpor was used to the greatest extent after weaning (post-lactation). Evidence is presented that the cost:benefit ratio for deep and prolonged periods of torpor may be highest during lactation. Microclimates of rock-crevice roosts mirrored the use of torpor throughout reproduction by bats. Lactation roosts (deeper, larger opening size) were more thermally stable and remained warmer at night compared to the shallow roosts used by pregnant and post-lactating females. It is shown that conclusions about relative use of torpor can differ depending on the units of comparison, necessitating measurement of all aspects of torpor (depth, duration and frequency). Comprehensive measurements, individual-based normothermic temperatures, and a definition of torpor that accounts for all energy savings, allow a more accurate depiction of patterns and facilitates inter-study comparisons.

Physical characteristics of roost sites used by the lesser long-eared bat Nyctophilus geoffroyi and Gould's wattled bat Chalinolobus gouldii were investigated in a farmland–remnant vegetation mosaic and adjacent forested floodplain in south-eastern Australia. A total of 45 individuals of N. geoffroyi and 27 C. gouldii were fitted with radio-transmitters, resulting in the location of 139 and 89 roosts, respectively. Male N. geoffroyi roosted in trees, fallen and decayed timber and artificial structures. These roosts were low to the ground, mainly under bark and in cracks in timber. Roosts of female N. geoffroyi were located higher above ground, and all within trees. Maternity roosts were predominantly located in large dead trees, approximately twice the diameter of roost trees used by females outside the breeding season. No maternity roosts were found under bark, despite half the roosts used by non-breeding females being located in these situations. Both sexes roosted primarily in dead timber and used cavities where the narrowest dimension of the entrance was 2.5 cm. Most roosts of C. gouldii were in dead spouts on large, live river red gum Eucalyptus camaldulensis trees. Intraspecific differences in roost characteristics were less pronounced for this species. Despite access to the same roosting opportunities, there were marked differences in roost selection between N. geoffroyi and C. gouldii. Both species favoured large diameter trees, but differed significantly for all other measured variables: type of roost structure, condition of roost tree (live or dead), height of roost tree, height of roost, and entrance dimensions. Although these species are among the most widespread bats in Australia and are often considered to be habitat ‘generalists’, both displayed a high level of discrimination in the roosts used. Clearly, roosting requirements are a complex and important issue in the conservation of even the most common species of bats.

Roost selection by tree roosting noctules Nyctalus noctula and Daubenton's bats Myotis daubentonii was determined by comparing roost characteristics with the characteristics of randomly chosen tree cavities in the area. The following parameters were determined for 81 noctule roosts and 27 Daubenton's bat roosts: tree species, type of cavity (woodpecker or natural), entrance height, compass direction of entrance and distance to the edge of the wood. Roost choice of both bat species was similar except that Daubenton's bats used woodpecker cavities less than noctules. Both bat species were selective in their roost choice. Daubenton's bats preferred oaks over beeches, and noctules preferred woodpecker cavities over natural cavities, indicating dependence on woodpeckers. Both bat species preferred to roost close to the edge of the wood. Roosting close to the edge of the wood may reduce commuting flight costs for both species or may be preferable for thermoregulatory reasons.

The selection of roost cavities by the threatened New Zealand long-tailed bat Chalinolobus tuberculatus was examined in unmodified temperate southern beech Nothofagus rainforest in Fiordland, New Zealand. We radio-tracked 73 bats during spring–autumn of 1993–1997 to 155 day roosts, all of which were in trees. One hundred and forty-nine roosts were in cavities: 84 in live trunks, 33 in dead trunks, 32 in large branches (seven in dead branches, 25 in live); and the remaining six (occupied by solitary bats) were beneath loose bark. We compared characteristics of the 84 roost cavities in live trunks with 57 random available cavities also in live trunks. Cavities used by C. tuberculatus were not a random subset of available cavities. In comparison to available cavities, bat roosts were all located in knot hole cavities, were high from the ground, and had little surrounding vegetation. All roosts were dry inside. Bats used a higher proportion of cavities which had medium sized entrances and internal cavities, and thicker cavity walls. A logistic regression model incorporating nine cavity variables classified roost and available cavities correctly 97% and 91% of the time, respectively. Distance to the nearest vegetation, cavity condition (wet or dry inside), height from the ground, and to a lesser extent internal cavity height, explained significant proportions of the variation between roost and available cavities. Bats changed roost sites virtually every day and it appeared that suitable cavities were abundant in the unlogged forests of the study area. It is unlikely that modified forests and forests managed for timber production contain such a large pool of potential roosting cavities.

We characterized the day roost sites of four pairs and one solitary tawny frogmouth Podargus strigoides in a woodland in south-eastern Australia. The birds were equipped with radio transmitters which enabled us to locate them daily from autumn 1997 to late summer 1998. Tree species, tree size, roost height and orientation of the roosting bird were recorded. Over the study period tawny frogmouths frequented a large number of day roosts (up to 71 per pair). Birds rarely used the same roost over extended time periods and most roosts were used for less than 3 days. Mature trees with a girth of more than 0.5 m were preferred as day roosts. Tawny frogmouths exhibited a significant preference for the coarse and dark-barked stringybark trees, but other tree species such as the smooth-barked, light-coloured gums were also frequented. However, when roosting in gum trees, dead branches were preferred, presumably as these have a coarser appearance than living branches and therefore provided better camouflaging. Especially during winter, the birds showed a significant selection of branches on the northern side of roost trees presumably to maximize sun exposure. During summer, two pairs maintained a significant northerly preference, whereas the others used roosts with random orientations. Small-scale seasonal movements in the area used for day roosting were also observed, with two pairs selecting a distinct area with a south-westerly aspect during summer which appeared to have less sun exposure. Our study suggests that tawny frogmouths select roosts to (1) minimize visibility from day predators and (2) to facilitate passive thermoregulation by sun-basking.