Population dynamics of little brown bats (Myotis lucifugus) at summer roosts: apparent survival, fidelity, abundance, and the influence of winter conditions

White-nose syndrome (WNS) has caused the death of millions of bats, but
the impacts have been more difficult to identify in western North America.
Understanding how WNS, or other threats, impact western bats may require
monitoring other roosts, such as maternity roosts and night roosts, where
bats aggregate in large numbers. Little brown bats (Myotis lucifugus) are
experiencing some of the greatest declines from WNS. Estimating survival
and understanding population dynamics can provide valuable data for
assessing population declines and informing conservation efforts. We
conducted a 5-year mark-recapture study of two M. lucifugus roosts in
Colorado. We used the robust design model to estimate apparent survival,
fidelity, and abundance to understand population dynamics, and
environmental covariates to understand how summer and winter weather
conditions impact adult female survival. We compared the fidelity and
capture probability of M. lucifugus between colonies to understand how
bats use such roosts. Overwinter survival increased with the number of
days with temperatures below freezing (β > 0.100, SE = 0.003), and
decreased with the number of days with snow cover (β < -0.40, SE
< 0.13). Adult female fidelity was higher at one maternity roost
than the other. Overwinter and oversummer adult female survival were high
(>0.90), and based on survival estimates and fungal-swabbing
results we believe these populations have yet to experience WNS. Recapture
of M. lucifugus using antennas that continuously read passive integrated
transponder tags allows rigorous estimation of bat population parameters
that can elucidate trends in abundance and changes in survival. Monitoring
populations at summer roosts can provide unique population ecology data
that monitoring hibernacula alone may not. Because few adult males are
captured at maternity colonies, and juvenile males have low fidelity,
additional effort should focus on understanding male M. lucifugus
population dynamics.