Predators that depend on patchily distributed prey face the problem of finding food patches where they can successfully compete for prey. While the competitive exclusion principle suggests that species can only coexist if their ecological niches show considerable differences, newer theory proposes that local coexistence can be facilitated by so-called stabilizing and equalizing mechanisms. A prerequisite to identify such mechanisms is the understanding of the strength and the nature of competition (i.e., interference or exploitation). We studied the interaction between two open-space foraging bats by testing if common noctule bats Nyctalus noctula shift their space use in response to simulated aggregations of conspecifics or heterospecific Pipistrellus nathusii. When confronted with playbacks of heterospecifics, N. noctula increased their activity in early summer, but decreased activity in late summer. This pattern was accompanied by a decrease in the proportion of large insects in late summer, suggesting a more intense competition for food in late compared to early summer. When confronted with playbacks of conspecifics, N. noctula did not change their activity, irrespective of season. Our results indicate that in early summer, intraspecific competition is more severe than interspecific competition for insectivorous bats. Likely, conspecifics engage in interference competition for flight space, and may suffer from reduced prey detectability as echolocation calls of conspecifics interfere with each other. During insect rich times, interspecific competition on the other hand may be mediated by fine scale vertical partitioning and the use non-interfering echolocation frequencies. In contrast, when food is scarce in late summer, bats may engage in exploitation competition. Our data suggests that N. noctula avoid aggregations of more agile bats like P. nathusii, probably due to impeded hunting success. Yet, as fast and efficient fliers, N. noctula may be able to escape this disadvantage by exploiting more distant foraging patches.
