Influence of investigator disturbance and temporal variation on surveys of bats roosting under bridges

Abstract

Given the importance of roost sites to bats, monitoring roost use is an important tool for assessing the local status of some species. We surveyed bridges for day-roosting bats in the Kisatchie National Forest of Louisiana to assess how temporal patterns of bridge use and disturbance from surveyors might affect survey results. We found no support for the hypothesis that surveys of day-roosts affected bat use of bridges. The 3 most common species roosting under bridges, Rafinesque's big-eared bats (Corynorhinus rafinesquii), eastern pipistrelles (Pipistrellus subflavus), and big brown bats (Eptesicus fuscus), exhibited strikingly different patterns of seasonal use. Because of seasonal variation in bridge use, a monitoring program would need to involve a minimum of a summer and a winter survey to document all common species that use bridges. Surveys during the day detected many more bats than surveys conducted at night. Although individuals tended to return to the same roosts over long periods of time, tagged bats often were absent during intervening surveys. Because individuals used multiple roosts, at least 3 surveys within a season will be necessary if an objective is to determine which specific bridges are used as roosts by locally abundant species.