Heterospecific attraction among forest birds: a review

In this paper we review the evidence for a habitat selection process where colonizing individuals use other species presence as cues to profitable breeding sites. Our experimental studies in Fennoscandia and North America have shown that den- sity and species richness of migrant birds breeding in the forests respond positively to experimentally augmented titmice densities. We used analytical modeling to analyze ecological conditions, which may favor a habitat selection process where later arriv- ing individuals (colonists) use the presence of earlier established species (residents) as a cue to profitable breeding sites. We compared the fitness of two colonist strate- gies: colonists could either directly sample the relative quality of the patches (termed samplers) or, alternatively, they could also use residents as a cue of patch quality (cue-users). Model results suggested that cue-using strategy is more beneficial in most ecological conditions and that this may result in heterospecific attraction. Fur- ther field experiments showed that migrant individuals selected nest sites at close vicinity of nesting titmice, and bred earlier and reproduced better. We conclude that heterospecific attraction may be a common and widespread process among forest birds particularly in seasonal environments.