Adaptive Significance of Aberrant Chickadee Song in Illinois

I studied the behavioral responses of Black-capped (Poecile atricapilus) and Carolina (P. carolinensis) chickadees to playback of Black-capped, Carolina, and aberrant chickadee songs throughout the state of Illinois. These two species are parapatrically distributed throughout the eastern United States, including Illinois. Some areas exist where the ranges of the two species overlap. Individuals in these contact zones often sing aberrant vocalizations. This study focused on the two parental vocalizations (i.e., Black-capped and Carolina song), and two of the most common aberrant vocalizations (the \u27Greenville\u27 and \u27Vandalia\u27 aberrant dialects), in an attempt to understand the adaptive significance of these aberrant songs. Playback experiments were conducted from March to June in 2011 and 2012 in 28 counties throughout Illinois. During playback trials, behavioral response data were collected and compared among the four different chickadee playback vocalizations; Black-capped, Carolina, and Greenville and Vandalia aberrant dialects. Black-capped and Carolina chickadees displayed their most aggressive responses to conspecific playback. However, contact zone individuals responded aggressively to all playback types. Aberrant vocalizations elicited largely intermediate responses from Black-capped and Carolina Chickadees. We found no differences in behavioral responses to playback when comparing responses of parental populations within 50 km and farther than 50 km from the contact zone. Due to the numerous aberrant dialects and parental songs that exist throughout the contact zones, it may be necessary for contact zone individuals to recognize and respond to most chickadee-like song types in order to establish and defend territories, and to attract mates. Intermediate responses to aberrant vocalizations by parental birds could be due to certain syllabic cues within the aberrant songs that elicit aggressive response