Bird Versus Bats: Behavioral Interactions at a Localized Food Source

During June and July 1976, we investigated the foraging behavior and interactions of bats and common nighthawks (Chordeiles minor) at a localized food source in Columbus, Ohio. The patch consisted of insects attracted to a cone of light produced by six large (1000 W) spotlights illuminating a sign. While foraging in the light cone members of both taxa engaged in both intra— and interspecific chases. Seventy—five percent of the bat—initiated and 94% of the nighthawk—initiated chases had nighthawk targets. The smaller bats appeared to dominate the larger birds in all individual aggressive encounters. Members of both taxa foraged differently while using the patch. When either birds or bats foraged in the absence of members of the other taxon, they foraged low in the light cone. When birds and bats were present simultaneously, the aggressive interactions conditioned their foraging behavior. When together, bats remained in the lower zone of the light cone, while some nighthawks rose into the upper zones. When foraging in these upper zones, nighthawks suffered a decrease in foraging efficiency based on energetic considerations. They missed more prey per attempted capture, and either increased the time between captures or increased their foraging speed in the upper zones. In any case, more energy was expended by nighthawks foraging in the presence of bats than when they were alone. The expanded pattern of patch used by the larger yet socially subordinate nighthawks in the presence of bats supports Morse's (1974) hypothesis. Morse predicted that social dominance would be more important than body size in determining resource use in communities where interspecific aggression and dominance is important in resource partitioning.