PLASTICITY IN ECHOLOCATION SIGNALS OF EUROPEAN PIPISTRELLE BATS IN SEARCH FLIGHT - IMPLICATIONS FOR HABITAT USE AND PREY DETECTION

We studied the echolocation and hunting behavior of three aerial insec tivorous species of bats (Vespertilionidae: Pipistrellus) in the field in order to characterize the signals used by the bats and to determin e how call structure varies in relation to habitat structure (''unclut tered'' versus ''cluttered'' space). We documented free-flying, natura lly foraging wild pipistrelles in various habitats using multiflash st ereophotography combined with simultaneous sound recordings. Then we r econstructed the bat's flight position in three-dimensional space and correlated it with the corresponding echolocation sequences. In all th ree species of pipistrelles, signal structure varied substantially. In echolocation sequences of the search phase we found a consistent asso ciation of signal types with habitat types. In uncluttered habitats (o bstacles more than 5 m from the bat) pipistrelles emitted almost exclu sively narrowband signals with bandwidths less than 15 kHz. In clutter ed habitats (obstacles less than 5 m from the bat) they switched to si gnals with bandwidths of more than 15 kHz. Wideband signals were also used when the bats were turning in cluttered and uncluttered spaces an d for an instant after turning away from obstacles. Prey detection occ ured only when the outgoing signal did not overlap with the returning echo from potential prey. The bats also avoided overlap of echoes from potential prey and obstacles. Based on the results of this study, we propose an overlap-free ''window'' within which pipistrelles may detec t potential prey and which allows predictions of minimum distances to prey and clutter-producing objects.