Monthly fluctuation of colony size and composition of the free-tailed bat Tadarida brasiliensis in the southernmost roost of Mexico

The free-tailed bat (Tadarida brasiliensis) likely migrates from the southwestern USA and northern Mexico to the south of Mexico to spend the winter in the Cueva San Francisco (CSF), which is the southernmost known roost of the species. In this study, we examined colony size and composition of the free-tailed bats at the CSF for two annual cycles to know if these parameters fluctuate throughout the year and whether these fluctuations indicate that the CSF is a winter roost. From February 2018 to November 2019, we conducted monthly observations and captured bats with a mist net of 12 × 2.6 m to determine sex, age, reproductive status, and relative abundance. The duration of the emergence flight (DEF) was also recorded. We detected significant fluctuations in colony size that were caused by the movement of pregnant females in spring, lactating and post-lactating females and males in summer, and by females and males in autumn and winter. The CSF was occupied all year-round by females and males. Our results indicate that the cave is a winter roost for females and males, a transitional roost for pregnant females that form their maternity colony elsewhere in summer, and a temporary roost for males in summer. We suggest that the CSF is crucial for the conservation of the free-tailed bat.