Negative indirect effects of an avian insectivore on the fruit set of an insect-pollinated herb

Though an abundance of research has focused on direct interactions between birds and plants, relatively few studies have reported on indirect interactions. Of those reports, all have focused on positive indirect effects of birds on plants through predation of plant natural enemies. We conducted an observational study along the Middle Rio Grande in New Mexico to determine if avian aerial insectivores had a negative, indirect impact on insect-pollinated plants through predation of insect pollinators. We found considerable taxonomic overlap, at the order and family level, between insects visiting sweet clover (Melilotus officinalis ) and those eaten by cliff swallows (Hirundo pyrrhonota ). We found a significant negative relationship between proximity of sweet clover to cliff swallow breeding colonies and sweet clover fruit set during the cliff swallow nestling period. The apparent effect of cliff swallows was strongest within 200 m of breeding colonies (approximately 50% reduction in fruit set) and decreased nonlinearly to a distance of approximately 400 m. Finally, we found that the clover fruit set gradient disappeared after the nestling period, when chicks had fledged and the colony was abandoned.