The effect of environmental variables on nightly activity patterns of insectivorous bats monitored over ten years in a tropical premontane forest, Costa Rica

ABSTRACT We investigated the role of environmental and temporal variables on the activity of 20 species of aerial insectivorous bats at a site in tropical premontane forest in Monteverde, Costa Rica. The primary study site was an opening in secondary forest adjacent to the Children’s Eternal Rainforest. We passively monitored the echolocation calls of all bats detected by an Anabat II detector all night for 1,147 nights from November 2000 through August 2010, from which we were able to identify the species in more than 250,000 bat passes. Simultaneously, we recorded environmental variables. Five species accounted for most of the activity, but the relative frequency of these species fluctuated widely over the ten-year period. The likelihood of any one of the three most common species being present was significantly influenced by wind, rain, phase of the moon, the time of night, the season of the year, and the presence of the other two species. In general, strong winds are associated with an increase in bat activity, moderate to heavy rain seems to halt bat activity, full moon appears to depress bat activity, and bats seem to be most active early in the evening. The number of bat passes at the study site declined over the ten-year period, possibly at least partially due to bat preference for foraging near bright lights, which increased markedly during the period due to development for tourism.