Sympatric song variant in mountain chickadees (Poecile gambeli) does not reduce aggression from black-capped chickadees (Poecile atricapillus)

Various species interact with one another daily and, when habitats overlap and species compete for resources, the interactions are often negative. Character displacements are shifts in traits that typically occur in regions of geographic overlap of closely related species. These shifts, which act to reduce negative interactions, can be behavioural, social or morphological. Previous research has shown that mountain chickadees (Poecile gambeli) have an altered song structure in regions of geographic overlap with the more dominant black-capped chickadee (Poecile atricapillus). Similar to the situation for European and Asian tits, mountain chickadees may have changed their song to decrease aggression from black-cappeds. To test this hypothesis, I conducted a playback study with black-capped chickadees in Prince George, BC in which I observed how they responded to the songs of mountain chickadees recorded in regions where the two species were (1) sympatric and (2) allopatric. I used principal component analysis to collapse behavioural response variables into a single “approach” variable and a single “vocalization” variable. I then used mixed-model analysis to determine whether there was a difference in approach or vocalization response to the two types of mountain chickadee songs. Black-capped chickadees responded with equal intensity to both types of mountain chickadee songs. My results demonstrate that mountain chickadee songs with the sympatric song variant do not reduce heterospecific aggression from black-capped chickadees. To my knowledge, this is the only instance of a character shift unassociated with reduced aggression in the family Paridae and raises interesting questions about the selective pressures leading to the evolution of this song divergence. It is possible that different selective pressures may result in similar evolutionary outcomes in the form of altered songs in sympatric populations