Experimental tests of selection against heterospecific aggression as a driver of avian color pattern divergence

Signal divergence is thought to reduce the costs of co-occurrence for closely related species and may thereby be important in the generation and maintenance of new biodiversity. In birds, closely related, sympatric species are more divergent in their color patterns than those that live apart, but the selective pressures driving sympatric divergence in color pattern are not well understood. Here, we conducted field experiments on naïve birds using spectrometer-matched, painted 3D-printed models to test whether selection against heterospecific aggression might drive color pattern divergence in the genus Poecile. We found that territorial male black-capped chickadees (P. atricapillus) are equally likely to attack sympatric and allopatric congeners, and wintering flocks are equally likely to visit feeders occupied by sympatric and allopatric congeners, despite sympatric congeners being more divergent in color pattern. These results suggest that either the concerted evolution of additional traits (e.g., discrimination), or interactions in sympatry that promote learning, are required if color pattern divergence among sympatric species is to reduce heterospecific aggression. Alternatively, color pattern divergence among sympatric species may be caused by other selective pressures, such as selection against hybridization or habitat partitioning and secondary signal adaptation