Structure and Dynamics of the Hybrid Zone between Black-capped Chickadee (poecile Atricapillus) and Carolina Chickadee (p. Carolinensis) in Southeastern Pennsylvania

—Analysis of the structure and stability of a hybrid zone can serve as a starting point for examining mechanisms that infl uence spatial and evolutionary relationships between species. Recent studies of the hybrid zone between Blackcapped Chickadee (Poecile atricapillus) and Carolina Chickadee (P. carolinensis) have suggested that genetic introgression is limited to a narrow zone, while also reinforcing the conclusion that the line of contact between these parapatrically distributed species is now shift ing northward. We investigated the structure, position, and recent movement of the chickadee hybrid zone in southeastern Pennsylvania. Using selectively neutral microsatellite DNA markers, along with mitochondrial DNA haplotypes, we documented large diff erences in genetic composition among three populations ≤65 km apart where we sampled large numbers of breeding residents during 1998–2003. Genetic results indicate that the three sites support a population of Carolina Chickadees (Great Marsh), a population in which most individuals exhibit evidence of hybridization (Nolde Forest), and a population comprising mostly Black-capped Chickadees but with evidence of hybridization now taking place (Hawk Mountain). The patt erns within the Nolde Forest population suggest that selection against hybrids may not be as strong as has been concluded from studies in other parts of the chickadee hybrid zone. Comparison of mitochondrial and nuclear genotypes between samples collected ∼15 years apart suggest that the northern edge of the hybrid zone shift ed by ∼20 km over this interval, with hybridization now occurring as far north as the Kitt atinny Ridge and beyond, where only Blackcapped Chickadee genotypes were previously detectable. Our data and historical accounts suggest that the hybrid zone, now ∼50 km wide, may have become wider while also shift ing northward. These results support the hypothesis that Carolina Chickadees enjoy a selective advantage during hybridization with Black-capped Chickadees, but both the proximate mechanisms and ultimate causes remain to be investigated. Received 8 December 2004, accepted 5 April 2006.