Selection of Maternity Roosts by Evening Bats (Nycticeius humeralis) in a Riparian Forest at the Northern Edge of Their Range

Flexibility in resource selection by a species at the edge of its geographic distribution is a key factor in determining the chance of successful range expansion. The evening bat (Nycticeius humeralis) is a medium-sized vespertilionid that is widely distributed over a span of 17 degrees of latitude, from the Gulf of Mexico northward to the Great Lakes in North America. In the core of their range, evening bats prefer mature and commonly available trees that are located close to water sources as maternity roosts. However, data are lacking on roost selection by this wide-ranging species in the periphery of its distribution. In this study, we examined roost selection of the evening bat at the northern edge of its continental range. We radio tracked 44 evening bats in Michigan, United States, and identified 33 maternity trees. We collected variables at individual tree, plot, and landscape scales and compared roost and randomly selected trees. Although evening bats preferred roosting in Fraxinus (ash), we found that the evening bat was a generalist in its choice of roost genera, as long as the tree receives sufficient solar exposure, which is presumably important at the cool, northern edge of its range. At the landscape level, evening bats favored roosts surrounded by a less dense canopy, which would provide easy access for a flying bat and allow considerable solar radiation to strike the tree for additional warmth. We also found that evening bats preferred forest sections with more forest-farmland edges. We suspected that evening bats at the northern edge of their range show less preference in roost selection than evening bats in the core of their range. This flexibility in roost selection could allow the evening bat to expand further north if summers become warmer and other tree-roosting species decline due to white-nose syndrome, thus providing competitive release.