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.