TIMING OF FORAGING FLIGHTS OF 3 SPECIES OF BATS IN RELATION TO INSECTACTIVITY AND PREDATION RISK

The flight activity of three species of insectivorous bats and their p rey was studied in north-east Scotland (57 degrees N) during summer 19 93. Aerial insects of sizes taken by bats were more abundant during th e day than during the night, but the highest abundance usually occurre d around dusk, partly reflecting increased flight of dipterans. In con trast, the flight activity of moths, mainly Geometridae and Pyralidae, was greatest around midnight. Two species of aerial-hawking bats, Pip istrellus 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, a nd subsequently foraged as their food was declining in abundance. In c ontrast, the foliage gleaning bat Plecotus auritus, which feeds primar ily on moths, did not emerge until about one hour after sunset, but wh ile the activity of its main prey was increasing. The two aerial-hawki ng bats therefore seem to be constrained from exploiting most of the e vening peak in aerial insect abundance, presumably because earlier eme rgence would result in higher predation risk at the higher light level s. P. auritus may have less to gain by emerging early, since it can fe ed on moths and non-flying prey independently of the activity of small insects at dusk. The conclusions have implications for the conservati on of bats and their habitats particularly at high latitudes. Protecti ve tree cover may allow earlier evening emergence of bats and therefor e provide access to more food.