Mechanisms and fitness effects of interspecific information use between migrant and resident birds

Interactions with potential competitors are an important componentof habitat quality. Due to the costs of coexistence with competitors,a breeding habitat selection strategy that avoids competitorsis expected to be favored. However, many migratory birds appearto gain benefits from an attraction to the presence of residentbirds, even though residents are assumed to be competitivelydominant. Thus far the mechanisms of this habitat selectionprocess, heterospecific attraction, are unknown, and the consequencesfor resident birds of migrant attraction remain untested. Throughheterospecific attraction, migrants may gain benefits if thedensity or territory location of residents positively reflectshabitat quality, and/or they gain benefits through increasedfrequency of social interactions with residents in foragingor predator detection. In this experiment, we examined the reciprocaleffects of spatial proximity on fitness-related traits in migrantpied flycatcher (Ficedula hypoleuca) and resident great tit(Parus major) by experimentally forcing them to breed eitheralone or in close proximity to each other. Surprisingly, greattits bore all the costs of coexistence while flycatchers wereunaffected, even gaining slight benefits. In concert with anearlier study, these results suggest that flycatchers use titsas information about good-quality nest-site locations whilebenefits from social interactions with tits are possible butless important. We suggest that utilizing interspecific socialinformation may be a common phenomenon between species sharingsimilar resource needs. Our results imply that the effects ofinterspecific information use can be asymmetric and may thereforehave implications for the patterns and consequences of speciescoexistence.