Concurrent changes in group size and roost use by reproductive female little brown bats (Myotis lucifugus)

Group formation is beneficial for many small endotherms, in part to create favourable conditions for developing offspring. However, for individuals occupying spatially limited structures, such as tree cavities, group formation may influence-and be influenced by-the range of structures available for use. This may be especially true of bats, which often form maternity groups that regularly change in size. We examined colonies of tree-roosting little brown bats (Myotis lucifugus (LeConte, 1831)) in northern Alberta to address two questions: (1) is group size associated with patterns of roost use, in particular the use of large-diameter trees, and (2) does group size change during reproduction and correspond to changes in roost use? We located roosts over 2 years using radiotelemetry and conducted exit counts at a subset of these roosts. Larger diameter trees had larger and more variable group sizes. Roost-tree diameter peaked near the start of parturition, the time when bats formed the largest groups. We suggest that large groups are especially beneficial at this time due to the thermoregulatory benefits to small, unfurred pups. Our results indicate that roost use and group size are interrelated and that large-diameter trees may be especially beneficial by supporting a wider range of group sizes.