VARIATION IN FORAGING ACTIVITY OF AN AERIAL INSECTIVOROUS BAT DURING REPRODUCTION

A maternity colony of northern bats, Eptesicus nilssonii, was observed regularly in southern Sweden (57-degrees-45-'N, 13-degrees-25'E) duri ng the reproductive period from 1988 to 1990. On average, females spen t ca. 3 h outside the roost each night. Time spent outside the roost d iminished during late pregnancy, when body weight was rapidly increasi ng. Foraging time was longer during lactation than during pregnancy. I t increased by > 100% between parturition and weaning, a reflection of increasing energetic costs of milk production. Foraging time did not differ significantly among individuals in the same stage of reproducti on. Non-breeding females did not show seasonal changes in foraging tim e. Low body weights and deferred breeding in younger females may be ef fects of limited opportunities for foraging or competitive relationshi ps among the females.