Extra-Pair Copulations in Black-Capped Chickadees: the Role of the Female

Abstract

Extra-pair copulation (EPC) behaviour was observed in three resident, colour-banded populations of black-capped chickadees. Because these populations were nonmigratory, I was able to determine the relative winter rank of all individuals involved (the females, their mates, and the other males) in the months prior to each breeding season. Over 14 years, a total of 13 EPCs, involving 12 females and 10 'other' males, were seen. Every one of the 13 EPCs was with a male ranked higher than the female's own mate. No female that was paired with an alpha male was ever seen engaging in an EPC. Only 4 observed EPCs occurred in the female's own territory; the other 9 all took place in the territory of the other male. Hence these females had apparently entered those territories to copulate with those resident males. All but one of the EPCs occurred within an hour of dawn, although WPCs are about equally likely to occur at any time of day. At least 4 of the observed EPCs occurred during the female's fertile period. No forced EPC attempts were seen. In resident species like chickadees, where females have the opportunity to learn the relative rank of all neighbouring males with respect to her own mate, females may regularly pursue the strategy of seeking out EPCs with superior partners. While lower-ranked males may suffer temporary losses through the EPCs of their mates, each male has some chance of attaining alpha rank if he lives long enough. Once at alpha rank, a male will likely engage in more EPCs, while having a mate that will no longer seek EPCs elsewhere.