Detection of prey in a cluttered environment by the northern bat Eptesicusnilssonii

We studied the acoustic behaviour of bats (Eptesicus nilssonii) hunting for large (wing span 5 cm) non-hearing hepialid moths (Hepialus humuli). Group s of silvery-white male H. humuli perform a short (30 min) hovering display flight over grassland at dusk. They typically hover at, or below, the tops of grass panicles and are therefore situated in a highly acoustically clut tered habitat. Occasionally, they move to a new position by making short (1 -5 s) flights at higher levels. E. nilssonii is not a clutter specialist, a nd yet we found that they attacked H. humuli within the 'clutter overlap zo ne'. The bats did not change their signal design in any marked manner for t his specific task. Measurements of echoes from a moth 10 cm above or below the grass tops showed that information for detecting the moths was availabl e to the bats. Nevertheless, the bats did not attack moths in stationary ho vering display flight, only when they moved above the grass panicles. The d uration of the up/down flights (movements) were almost always longer than a n entire capture sequence by the bats. Apparently, the bats rely on the mov ement of the moth in space, monitored over successive echoes, to discrimina te moth echoes from overlapping clutter echoes.