Natural History of the Brazilian Free-Tailed Bat in the San Luis Valley of Colorado

Abstract

The natural history of Brazilian free-tailed bats, Tadarida brasiliensis mexicana (Saussure), in an atypical, predominantly male colony in south-central Colorado was investigated in 1982 and 1983. The colony inhabits the Orient Mine at the base of the Sangre de Cristo Mountains in the San Luis Valley. Iron ore was removed from the mine (elevation, 2,800–3,000 m) between 1881 and 1932. Colonization by free-tailed bats occurred no earlier than 1900 and most likely after the mine closed in 1932. The colony roosts in a cavernous underground stope in which ambient temperature varies from 6 to 12°C when bats are present from mid-June through October. At peak numbers, the colony consists of more than 100,000 bats. Outflights in both years lasted an average of nearly 1 h. Bats emerged earlier relative to sunset in late summer and autumn than in early summer. Timing of outflights was not affected by percent cloud cover. Outflights were dispersed in June and became more concentrated, often with serpentine components, later in the year. Capture samples taken with mist nets indicated that composition of the colony was 98% adult males until mid-August, when adult females and young-of-the-year became more common.