Calls in the Forest: A Comparative Approach to How Bats Find Tree Cavities

Abstract

Although tree cavities are a particularly critical resource for forest bats, how bats search for and find new roosts is still poorly known. Building on a recent study on the sensory basis of roost finding in the noctule (Ruczynski et al. 2007), here we take a comparative approach to how bats find roosts. We tested the hypothesis that species' flight abilities and echolocation call characteristics play important roles in how well and by which cues bats find new tree roosts. We used the very manoeuvrable, faintly echolocating brown long-eared bat (Plecotus auritus) and the less manoeuvrable, louder Daubenton's bat (Myotis daubentonii) as study species. The species are sympatric in European temperate forests and both roost in tree cavities. We trained bats in short-term captivity to find entrances to tree cavities and experimentally manipulated the sensory cues available to them. In both species, cue type influenced the search time for successful cavity detection. Visual, olfactory and temperature cues did not improve the bats' performance over the performance by echolocation alone. Eavesdropping on conspecific echolocation calls played back from inside the cavity decreased search time in Daubenton's bat (M. daubentonii), underlining the double function of echolocation signals - orientation and communication. This was not so in the brown long-eared bat (P. auritus) that has low call amplitudes. The highly manoeuvrable P. auritus found cavities typically from flight and the less manoeuvrable M. daubentonii found more entrances during crawling. Comparison with the noctule data from Ruczynski et al. (2007) indicates that manoeuvrability predicts the mode of cavity search. It further highlights the importance of call amplitude for eavesdropping and cavity detection in bats.