Factors influencing foraging time in two aerial insectivores: the bird Chordeiles minor and the bat Eptesicus fuscus

An energetic model was used to analyze factors important in determining foraging time in two species of aerial insectivore, the bird Chordeiles minor and the bat Eptesicus fuscus. Our results suggest that E. fuscus and C. minor, though feeding at the same site and taking predominantly the same prey, use different foraging strategies and their foraging times are limited by different factors. Chordeiles minor forage for relatively short periods. We predict that this species may attack more than one prey item simultaneously. As predicted, it seems likely that foraging time of the birds is limited by the duration of twilight. Foraging bouts tend to be short and remain relatively constant in duration throughout the summer. Eptesicus fuscus, on the other hand, is not limited by the duration of twilight, but alters its foraging time in response to changes in its energy requirements, although other factors such as the duration of prey activity periods also have an effect. The abundant availability of prey at our study site meant that foraging time was determined primarily by the animals' daily energy demands.