Detection of prey in a cluttered environment by the northern bat Eptesicus nilssonii.

We studied the acoustic behaviour of bats (Eptesicus nilssonii) hunting for large (wing span 5 cm) non-hearing hepialid moths (Hepialus humuli). Groups 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 cluttered 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, and yet we found that they attacked H. humuli within the 'clutter overlap zone'. The bats did not change their signal design in any marked manner for this specific task. Measurements of echoes from a moth 10 cm above or below the grass tops showed that information for detecting the moths was available to the bats. Nevertheless, the bats did not attack moths in stationary hovering display flight, only when they moved above the grass panicles. The duration of the up/down flights (movements) were almost always longer than an entire capture sequence by the bats. Apparently, the bats rely on the movement of the moth in space, monitored over successive echoes, to discriminate moth echoes from overlapping clutter echoes.