The Brazilian free-tailed bat, Tadarida brasiliensis, roosts in very large colonies, consisting of hundreds of thousands of individuals. Each night, bats emerge from their day roosts in dense columns in a highly coordinated manner. We recorded short segments of an emergence using three spatially-calibrated and temporally-synchronized thermal infrared cameras. We applied stereoscopic methods to reconstruct the three-dimensional positions of these flying bats. We applied a multiple hypothesis tracking algorithm to obtain 7,016 reconstructed trajectories. Our analysis includes estimates of the velocities of bats in flight, the distances between animals within the emergence column, and the angles subtended by the bats and their nearest neighbors. 2. Methods We analyzed the nightly emergence behavior of a colony of Brazilian free-tailed bats at a cave in Texas. We used three thermal infrared FLIR SC6000 cameras that recorded 16 bit video at a resolution of 640 x 512 pixels at 125 frames per second. Example frames of video containing approximately 100 bats are shown in Figure 1. 1
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