Heterospecific attraction affects community structure and migrant abundances in northern breeding bird communities

We experimentally tested the heterospecific attraction hypothesis by manipulating densities of resident birds (black-capped chickadee, Parus atricapillus, red-breasted nuthatch, Sitta canadensis, and white-breasted nuthatch, S. carolinensis) on 7 lake islands in northern Minnesota, U.S.A. Resident numbers were increased by winter feeding and decreased by removals using mist nets. The heterospecific attraction hypothesis states that migrants use residents as a cue to identify the best sites for breeding in a heterogeneous landscape because residents have already selected higher quality sites. The hypothesis predicts that there will be a higher overall density of migrants associated with increased resident density. Results showed that there was a consistent change in migrant bird community structure (i.e., in the relative abundance of species) in response to manipulation. This change resulted mainly from an increased density of arboreal insectivores, i.e., species that belong to the same foraging guild as residents, suggesting that food is an important factor contributing to heterospecific attraction. The red-eyed vireo, Vireo olivaceus, and black-and-white warbler, Mniotilta varia, showed the strongest positive response. No migrant species responded negatively to augmented resident density, suggesting that interspecific competition may be less important in structuring breeding bird communities in northern forests. Our results support the view that positive interactions should be considered when studying community structure.