Feeding activity of the northern bat Eptesicus nilssoni during pregnancy and lactation.

Female northern bats emerged at dusk to feed when aerial insect density was 0.1 m-3 or higher. A model suggests that this is the threshold level above which foraging may be energetically profitable. Insect density was determined mainly by air temperature, and reached the threshold level at 6-10°C. Mean temperature during early summer in six years was correlated with the date of parturition. This suggests that periods when foraging was not energetically profitable were spent in torpor.