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Stability in distributions of black-capped, Carolina, and aberrant chickadee song types in Illinois.

Article Excerpt
Hybrid zones develop where related, largely parapatric taxa overlap and interbreed, resulting in a population that is at least partially comprised of hybrids (Futuyma 1986). Morphological, genetic, and behavioral traits may be affected by interactions occurring in hybrid zones (Robbins et al. 1986, Hewitt 1989, Curry 2005, Curry et al. 2007). The shape and stability of hybrid zones is influenced by the spatial relationships of habitat types, and by the specific adaptations, behavioral interactions, and genetic compatibility of the taxa (Hewitt 1989, Rohwer and Wood 1998). Hybridization of songs of oscine songbirds, in addition to genetic hybridization, may occur between populations of conspecifics or heterospecifics (Chilton and Lein 1996, Martens 1996, Sattler et al. 2007).

Black-capped (BCCH) (Poecile atricapillus) and Carolina (CACH) (P. carolinensis) chickadees are closely related species which are extremely similar in morphology, behavior, and ecology (Brewer 1963, Rising 1968, Merritt 1978). They are parapatrically distributed from New Jersey to Kansas. BCCH inhabit most of northern North America and higher elevations in the Appalachians. CACH are limited to the southeastern United States and lower elevations in the Appalachians. Narrow hybrid zones occur sporadically along this boundary; Illinois is one of nine states where both BCCH and CACH regularly occur (Brewer 1963, Rising 1968, Merritt 1978, Robbins et al. 1986, Grubb et al. 1994, Sattler 1996, Bronson et al. 2005). Molecular evidence from these areas indicates these species hybridize frequently in areas of sympatry (Robbins et al. 1986, Sawaya 1990, Sattler 1996, Sattler and Braun 2000, Reudink et al. 2007), despite not being sister species (Gill et al. 1993, 2005). Recent studies in Pennsylvania and Ohio have documented significant northward shifts in hybrid zones, at least in these areas (Bronson et al. 2005, Reudink et al. 2007). It is unclear whether this pattern extends across the entire length of the range interface.

The songs of CACH and BCCH are similar. The BCCH song consists of two whistled notes with frequencies below 5 kHz (Fig. 1B). Little variation in BCCH song exists over their entire range (Kroodsma et al. 1995, 1999). CACH songs usually contain four whistled notes (Fig. 1A), but at times include more (Ward 1966). Field studies by Rossano (2003) and Sattler et al. (2007) found that song type in chickadees does not always match their genetic identity (Kroodsma et al. [1995] reported both species can learn each other's song in an aviary setting), but vocal admixture (including bilingually singing birds and songs with intermediate or atypical characteristics) seems restricted to locations at or near the range interface. Genetic introgression, however, has been found to extend much farther (Sattler et al. 2007).

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Brewer (1963) mapped the distributions of these species in Illinois from 1954 to 1959 and the areas of contact between them. He recorded their distributions by mapping the occurrences of whistled songs typical to each species, bilingual individuals, and "hybrid" song types. He described two hybrid zones in Illinois, one in Bond County and one in Fayette County. We remapped the distributions in 1997-2000 of, and hybrid zones between, BCCH and CACH song types in Illinois. Our primary objective was to compare their distributions documented in this study with areas of sympatry, and the distributions of parental song-types described and mapped by Brewer (1963). We predicted, based on previous studies, the Illinois contact zone had shifted northward significantly.

METHODS

Field Protocols.--We recorded chickadee vocalizations at 185 sites in 15 Illinois counties. Most songs were collected from 17 April to 16 July 1999. Additional...

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