Extrapair paternity and mate choice in a chickadee hybrid zone

The dynamics of hybrid zones are likely to be influenced greatly by patterns of mate choice, including ‘‘cryptic’’ choice mediated through extrapair copulations. To understand changes in hybrid zones over time and space, a detailed examination of mating patterns and correlates is needed. We studied the role of extrapair fertilizations (EPFs) in the breeding biology of hybridizing black-capped and Carolina chickadees in southeastern Pennsylvania over 4 years, using microsatellite DNA markers. We detected extrapair offspring (EPO) in 56% of 90 broods examined; these accounted for at least 26% of 477 offspring. Chickadees do not appear to use EPFs to reduce costs of heterospecific pairing: EPFs were no more likely to occur in genetically dissimilar (heterospecific) social pairs than in pairs where social mates were genetically similar. However, females paired with black-capped–like males were more likely to have EPO. Females that acquired EPFs did not obtain these from males genetically similar to themselves; instead, all females, regardless of their genotype or that of their social mate, tended to prefer Carolina-like males as extrapair partners. There was no relationship between the presence of EPO and hatching or fledging success. High rates of extrapair paternity and apparent female preference for Carolina-like males suggest that mate choice is an important influence in ongoing northward movement of this hybrid zone.