Breeding synchrony and EPF rates: the key to a can of worms?

G iven that roughly 95% of bird species are classified as 'monogamous' 1 , some might claim that the class Aves is short on mating system diversity. Whatever the group might lack in this respect, however, is offset by rather impressive species differences in the prevalence of covert matings by females. By a recent count 2 , extra-pair fertilization (EPF) rates range from 0 to 76% across avian species, with the mean for all species at about 13%. One explanation for why EPF rates vary so much among birds is gaining momentum. In new comparative analyses of passerines, Stutchbury 3,4 presents evidence that breeding synchrony can be a key element: the more synchronous the breeding, the greater the percentage of females whose broods contain at least one offspring sired by an extra-pair mating. Stutchbury and Morton et al. 5 , in another new paper, argue that the pattern is a consequence of the enabling effects of synchrony on female choice of extra-pair partners: in synchronously breeding species, males engage in sexual display simultaneously, so females, at relatively low cost to themselves, are better able to detect differences among potential mates, and thus better able to select extra-pair mates of higher genetic quality. Alternative perspectives on the relationship between EPF rates and syn-chrony are abundant and provocative. Westneat et al. 6 have proposed that, if extra-pair copulations (EPCs) are predominantly male-initiated, then breeding synchrony should be negatively related to EPF rate, the opposite of Stutchbury's result. Their logic was that synchrony imposes seasonal restrictions on the availability to males of fertilizable EPC partners, and it promotes greater parity in the ratio of copulation-seeking males to fertilizable females. Furthermore, males might have fewer opportunities to pursue EPCs when breeding is synchronous because their time is occupied with mate-guarding and/or sexual behavior with their own mates at the very time when the mates of other males are fer-tilizable 7. Weatherhead and Yezerinac 8 have argued that Stutchbury's result may well be an artifact of some third variable that covaries with synchronous breeding. For example, synchronous breeding (fertile periods) might be preceded by nearly simultaneous establishment of territories by males and rapid settlement by females. These earlier limitations might leave brief opportunity for comparisons, causing females to postpone their choice of genetic sires for their young until after they have chosen a place to live. Weatherhead and Yezerinac 8 further reason that if Stutchbury …