During the summer months as many as 3 million Brazilian free-tailed bats
(Tadarida brasiliensis mexicana) reside in an abandoned train tunnel at the Old Tunnel
Wildlife Management Area, Kendall County, Texas. This study examined the population
ecology of juvenile bats as it related to emergence patterns, sex ratios, and recruitment.
To collect large numbers of bats efficiently a 1.8 m by 2 m double frame Harp trap was
lowered in front of the south opening of the tunnel shortly after the start of the
emergence. Banding occurred in late July and August of 1999 and 2000. Bands were
placed on 4,000 and 3,625 bats, respectively. The sex of each juvenile banded and the
dates of banding were recorded. Capture data showed no significant difference in the sex
ratio in 1999, however the ratio was significantly male biased (p < 0.001) in 2000. Data
from recaptures were analyzed using the Jolly-Seber method to estimate the juvenile
population for both summers. Results show a decrease in the population from 1999 to
2000. Population trends, however, appear to be similar between the two summers.