Brazilian free-tailed bats (Tadarida brasiliensis: Molossidae, Chiroptera) at high altitude: links to migratory insect populations

Existing information on the activity of bats in the aerosphere is restricted almost exclusively to altitudes that are within a few tens of meters above the ground. We report a total of 50.2 h of ultrasonic recordings made using radio microphonic bat detectors suspended from free-floating helium balloons and from kites. The data include a total of 22 353 echolocative calls from ground-level to 1118 m above ground level (AGL). These calls are attributed to Brazilian free-tailed bats based on acoustic features and the large numbers and high-altitude aerial dispersion of these bats over the local landscape. Bat activity varied significantly throughout the air column and was greatest at 400-500 m AGL and near ground level. Feeding buzzes, indicating feeding on aerial prey, were most abundant near ground level and at 400-500 m, and were detected to altitudes of ∼ 900 m AGL. The peak activity of bats at 400-500 m AGL is concordant with the altitude of the atmospheric boundary layer and the seasonal formation of the low-elevation southerly wind jet that has been identified as a major aeroecological corridor for the nocturnal dispersal of noctuid moths and other insects.