Seasonal Emergence Behavior of Brazilian Free-tailed Bats at Carlsbad Caverns
Department of Biology
Department of Computer Science
Last modified: August 20, 2007
Brazilian free-tailed bats (Tadarida brasiliensis) form some of the largest aggregations of non-human mammals known to mankind. The estimated nightly emergence of hundreds of thousands to millions of this species from Carlsbad Caverns and other caves in the southwestern United States has challenged scientists to accurately and reliably access colony size. However, roost accessibility, cave topography, variable roosting densities, and nocturnal behavior have made colony estimates difficult, often resulting in highly variable and sometimes unrealistic estimates. Recorded changes in colony size may reflect natural patterns of behavior, disturbances from anthropogenic factors, and/or from different and/or inaccurate census methods. Previous reports indicate a general decline in colony size. Understanding the ecological and economic importance of this species requires reliable estimates of numbers and composition of the colony. Nightly emergences were monitored monthly as bats emerged at dusk, using high-performance thermal infrared cameras from March through October in 2005. Recordings were analyzed using adaptive visual recognition and tracking algorithms that can automatically identify and track individual bats. In addition, infrared thermal cameras were deployed in the interior of the cave to monitor the activity of bats throughout the night. We present the first accurate and reliable estimates of the number of Brazilian free-tailed bats at Carlsbad Caverns. Our results indicate large fluctuation in the size of the colony on a seasonal as well as daily basis. Some of these fluctuations reflect seasonal migratory habits of these bats, but others may be linked to large-scale seasonal weather patterns and related insect availability.