As income breeders, lactating female bats rely on current resource intake to support costs of reproduction and so must reconcile the conflicting demands of foraging and nursing. We documented changes in the movement of female little brown bats (Myotis lucifugus) around roosts between pregnancy and lactation. Home-range size dropped by 51% between pregnancy and lactation, resulting in a 35% decrease in flight distances. Although pregnant females rarely returned to roosts during the night, lactating females returned 1–2 times, which led to an increase in activity at the roosts beginning about 3 h after initial emergence. We argue that their high mass-specific milk production forces lactating females to nurse at night, which in turn imposes a constraint on foraging distances. The shift to a smaller home range is probably facilitated by the concomitant increase in insect biomass during the July lactation period.