Chickadees sing different songs in sympatry versus allopatry.

This article investigates character displacement in the songs of two closely related and ecologically similar songbirds, black-capped chickadees (Poecile atricapillus) and mountain chickadees (Poecile gambeli), which coexist in sympatric and allopatric populations. The study finds that mountain chickadees in sympatry sing songs that differ significantly from allopatric populations and sympatric black-capped chickadees, producing more notes, often including two introductory notes, and exhibiting a smaller glissando in the first note. These asymmetric song divergences in the subordinate mountain chickadees are consistent with theories of reproductive and agonistic character displacement, potentially functioning to reduce costly interspecific aggression and maladaptive hybridization, both of which impose fitness costs in sympatry. The findings highlight the role of learned vocal signals in maintaining species boundaries where closely related species co-occur and suggest that song divergence may serve as a reproductive barrier in this system.