Timing and variation in the emergence and return of Mexican free-tailed bats, Tadarida brasiliensis mexicana

We investigated the timing and variation in evening emergence and dawn retu rn of a maternity colony of Mexican free-tailed bats, Tadarida brasiliensis mexicana, in the summers of 1996 and 1997. In both summers, the onset of e vening emergences and the end of dawn returns correlated with sunset and su nrise, respectively. Bats tended to emerge earlier in relation to sunset du ring late summer than in spring to early summer. Dawn returns ended progres sively later in relation to sunrise through the season. Bats emerged much e arlier and ended dawn returns later in 1996 than in 1997, coincident with a severe drought in spring to midsummer 1996. Bat captures during different seasons at the cave entrance indicate that patterns of emergence and return were non-random with regard to age, sex, and reproductive classes of bats. Higher proportions of reproductive females emerged earlier in the evening and returned later at dawn. Non-reproductive and post-lactating females and males exhibited an opposite pattern. Higher proportions of juveniles tende d to emerge earlier in the evening and to return earlier at dawn.