Timing of Foraging Flights of Three Species of Bats in Relation to Insect Activity and Predation Risk

Abstract

The flight activity of three species of insectivorous bats and their prey was studied in north-east Scotland (57°N) during summer 1993. Aerial insects of sizes taken by bats were more abundant during the day than during the night, but the highest abundance usually occurred around dusk, partly reflecting increased flight of dipterans. In contrast, the flight activity of moths, mainly Geometridae and Pyralidae, was greatest around midnight. Two species of aerial-hawking bats, Pipistrellus pipistrellus and Myotis daubentonii, which feed primarily on small flying insects, mainly Diptera, emerged from their roosts 15-30 min after sunset, during or after the dusk peak in insect activity, and subsequently foraged as their food was declining in abundance. In contrast, the foliage gleaning bat Plecotus auritus, which feeds primarily on moths, did not emerge until about one hour after sunset, but while the activity of its main prey was increasing. The two aerial-hawking bats therefore seem to be constrained from exploiting most of the evening peak in aerial insect abundance, presumably because earlier emergence would result in higher predation risk at the higher light levels. P. auritus may have less to gain by emerging early, since it can feed on moths and non-flying prey independently of the activity of small insects at dusk. The conclusions have implications for the conservation of bats and their habitats particularly at high latitudes. Protective tree cover may allow earlier evening emergence of bats and therefore provide access to more food.