VARIATION IN THE SONAR OF AN AERIAL-HAWKING BAT (EPTESICUS-NILSSONII)

Echolocation calls from 10 individually marked female northern bats (E ptesicus nilssonii) were recorded as the bats foraged at three distinc t feeding sites (territories) near their maternity roost in southern S weden (57-degrees-N). In addition, recordings of unmarked bats were ma de in northern Sweden (65-degrees-N). The frequency at maximum amplitu de of 'search phase' echolocation pulses was bimodally distributed, wi th peaks around 29-30 kHz and 31-32 kHz and was negatively correlated with pulse duration. The frequency at maximum amplitude was related to flight altitude (bats used higher frequencies when they flew near the ground) and also differed among the feeding sites. Hence, much of the variation, probably including regional differences, was behavioural a nd is interpreted as short term (in the order of s or min) adaptation to current foraging situations. Variation among individual bats, cause d by age and size, seemed to be of less importance. Individuals did no t use exclusive frequency bands.