Character displacement in dawn chorusing behaviour of sympatric mountain and black-capped chickadees

Signals of closely related species tend to be more distinct when occurring in sympatry than in allopatry. Such differences allow species-specific identification and presumably reduce interspecific mating. Among chickadees, dawn chorus behaviour affects female mate choice. Within our sympatric study population of black-capped chickadees, Poecile atricapillus, and mountain chickadees, Poecile gambeli, directional hybridization occurs via extrapair matings between female mountain chickadees (the subordinate species) and male black-capped chickadees (the dominant species). In this study, we compared dawn singing from this sympatric population with dawn recordings from allopatric populations of each species. Mountain chickadees used more chick-a-dee calls than songs during the dawn chorus when they co-occurred with black-capped chickadees in the sympatric population, but used similar proportions of calls and songs in the allopatric populations. We also found differences in the fine structure of the song; both species typically had a descending first note in their song (glissando), but mountain chickadees in the sympatric population used an ascending first note. The internote ratio between the first two notes of the song of the sympatric mountain chickadees lacked a characteristic frequency drop found in the allopatric mountain chickadee population and in the allopatric and sympatric populations of black-capped chickadees. Geographical analysis of songs of mountain chickadees across western North America revealed consistent differences in song features among sympatric/allopatric populations in different regions, but the nature of character shifts were not always parallel among populations. These findings illustrate possible character displacement in a subordinate species (mountain chickadee) to reduce acoustic overlap with a dominant heterospecific (black-capped chickadee).