Patterns and Implications of Aberrant Singing Behaviors of Black-capped (Poecile atricapillus) and Carolina (Poecile carolinensis) Chickadees in Illinois

I studied the singing behavior of the closely related Black-capped (Poecile atricapillus) and Carolina (Poecile carolinensis) Chickadees along their range interface in central Illinois. These sibling species are parapatrically distributed from the Appalachians through the Midwest to Kansas. Where their ranges do overlap, small hybrid zones form and chickadees in these areas frequently sing aberrant songs. The majority of the songs studied were recorded 17 April-16 July 1999 in fifteen counties throughout central Illinois. I measured 8 frequency and duration characteristics of the first two notes of songs that began with two whistled notes. The averages of these characteristics for each song type at each site were classified as Black-capped Chickadee (BCC), Carolina Chickadee (CC) or intermediate on the basis of a discriminant function analysis. I developed a set of objective spectrographic criteria (OSC) that used the frequency, also duration, and basic structural characteristics of the entire song to classify them as BCC, CC, or aberrant. Based on the distribution of intermediate and aberrant song types I located four contact zones in central Illinois between these chickadees. The largest was located in Bond, Fayette, and Montgomery counties. There were smaller contact zones in Shelby, Douglas, and Champaign counties. A comparison among the distributions of BCC, CC and aberrant chickadee singing now and during a previous study in Illinois (1954-59) revealed that there has been little or no change in these two species\u27 distributions in the past 40 years. The stablility of the distributions of the song types associated with each species most likely has resulted from the fact that the factors that determined the relative distributions of these species (e.g., habitat availability) have remained stable. The greatest diversity of song types was present in the largest contact zone. I found three unique dialects within the contact zones, one each in Bond, Fayette, and Shelby counties. The Vandalia dialect (Fayette Co.) is probably at least 40 years old (Brewer 1959). I used the song types present at each site to sort the sites into eight repertoire categories based on the overall combination of song types present. Repertoire analysis revealed that bilingual singing was widespread and that individuals that sang both CC and aberrant song types that contained high frequency (\u3e6kHz) whistled notes or both BCC and songs containing only low frequency (≤5kHz) whistled notes were concentrated in specific, mutually exclusive areas. The presence of patterns in the distribution of aberrant song types and repertoire constitution in these contact zones suggests that the interaction of BCC and CC has led to the development of unique song cultures in at least some contact zones