Use of passive integrated transponder tags to assess movements and demography of Townsend's Big-eared Bat (Corynorhinus townsendii) in Colorado.

Abstract

Determining bat seasonal roost use, demographic rates (fidelity and survival), and movements are necessary metrics to rigorously evaluate population status and trends. Little is known about these metrics for Townsend's big-eared bats (Corynorhinus townsendii), a state and federal sensitive species, in Colorado. Therefore, our objectives were to monitor daily use of a large maternity colony (∼ 900 bats) inside a gated mine from 2011 to 2025 by marking bats with PIT tags (n = 840), evaluate their tolerance of the disturbance, assess seasonal patterns, and estimate demographic rates for this species. Bats tagged as adults were regularly detected to reach 10 yr of age, with at least 1 individual reaching a minimum of 15 yr of age. Despite concerns of capture and tagging disturbance for this species, 90% of individuals were detected again at the maternity colony at least once during the same summer of tagging. We detected 46% of females and 41% of males at least once across subsequent years following tagging. Variation by age was apparent with juveniles detected during at least 1 subsequent maternity season at lower percentages than adults. Numbers of tagged individuals present varied seasonally but the pattern of seasonal variation was consistent across years. We confirmed autumn movement to a known hibernaculum in a cave 48 km north of the maternity roost for 31 tagged bats, indicating that summer colonies fragment for winter rather than moving as an intact group. These data represent the first documented summer-to-winter movement of PIT-tagged C. townsendii and suggest that short-distance, localized migrations to hibernacula are an over-wintering strategy for this species where resources permit. We documented loss of PIT tags by C. townsendii and accounted for it by generating minimum tag retention estimates. Apparent annual survival for adult females across all years was 0.74 (95% CI = 0.60-0.87), similar to that for other PIT tag studies of bat species. Adult females and juvenile males showed high site fidelity with yearly return probabilities above 0.95, whereas estimates for juvenile females were lower (0.80). Our data show that PIT tags can be used to mark and monitor C. townsendii safely, providing managers with rigorous demographic data that can guide management actions.