The Dynamics of Vocal, Morphological and Molecular Interaction between Hybridizing Black-Capped and Carolina Chickadees

Previous investigation of genetic interactions between black-capped and Carolina chickadees (Parus atricapillus and P. carolinensis) has been hindered by their morphological similarity, and by a paucity of differentiated genetic markers distinguishing them. Nine fixed or strongly differentiated restriction fragment length polymorphism (RFLP) markers were developed, and one strongly differentiated allozyme locus was detected. These markers were used in conjunction with one fixed allozyme marker and three fixed RFLP markers previously available for these birds to examine interactions along their contact zone at three locations. A principal component analysis of mass, wing length and tail length revealed minimal morphological intermediacy at the contact zone in Virginia, in contrast with more extensive intermediacy at the contact zone in West Virginia, despite high levels of hybridization at both locations. This reflects the unreliable nature of these morphometric characters in reflecting genetic interactions occurring along this hybrid zone, due to the poor morphometric resolution of P. atricapillus and E. carolinensis. Principal component and discriminant analysis of eight frequency and note duration variables showed songs of intermediate nature to be present only at the contact zone in Missouri, while bilingual singing was widespread both in Missouri and West Virginia, but limited in Virginia. The proportion of hybrids detected by the diagnostic genetic markers was high at all three of these regions, demonstrating that like morphology, use of song is unreliable in assessing genetic interactions between E. atricapillus and P. carolinensis. Heterospecific song learning between these chickadees is a potential explanation for this result. Introgression of mitochondrial DNA across the hybrid zone was limited relative to autosomal introgression at all three locations. This observation is consistent with the potential operation of Haldane\u27s rule in F$\sb1$ hybrids. Introgression of sex-linked markers was likewise limited, suggesting that epistatic interactions involving sex-linked genes contribute to reproductive isolation between these chickadees. In contrast, introgression at autosomal loci appears to be more substantial overall, reflecting the semipermeable nature of this hybrid zone. A correlation between allele frequency and elevation suggests that ecological factors are also important to this hybrid zone\u27s dynamics