Depending on the circumstances under which extra-pair mating occurs, theory makes opposing predictions about how reproductive synchrony should influence extra-pair paternity. This study investigated which sex initiated extra-pair mating in the yellow warbler, whether extra-pair behaviour and extra-pair paternity were related to reproductive synchrony, and whether synchrony affected the mating success of all males equally. Observations and captures of individuals making territorial intrusions indicated that males initiated extra-pair mating. Spatial patterns of male territorial intrusions and of extra-pair paternity were similar. Males’ extra-pair activity was reduced when their social partner was fertile. More offspring were sired by the extra-pair male when the female nested asynchronously with that sire's social mate. Neither population-wide synchrony, nor synchrony with neighbours, however, seemed to predict the incidence of extra-pair parentage or the identity of the sire, indicating factors other than synchrony were also important. Males with more breast streaking (a plumage ornament) were more successful as extra-pair sires, and were least affected by the constraint of synchrony. Larger males were less often cuckolded, and achieved extra-pair success mainly when their partner was not fertile. Thus, male yellow warblers apparently use different mating tactics depending on their plumage and size. More generally, the results suggest how mating strategies are affected by which sex initiates extra-pair mating and by the relative contributions of within-pair and extra-pair paternity to total reproductive success.
From 1992 to 1994 we studied yellow warblers that bred in three sites between 2 and 8 km apart near the Queen’s University Biological Station in eastern Ontario, Canada (44 34'N, 76 20'W; described in Yezerinac et al. 1995). Each site was surrounded by habitat in which yellow warblers neither nested nor held territories. During the three breeding seasons, respectively, 90, 95 and 100% of territorial males, and 32, 61 and 78% of nesting females were banded and had 10–100 μl of blood taken from the brachial vein. At capture, we measured the mass of birds to the nearest 0.1 g, tarsus length to the nearest 0.1 mm and length of the unflattened wing chord to the nearest 0.5 mm. We videotaped each bird to measure the proportion of the breast covered by reddish streaking (as described in Yezerinac & Weatherhead 1997). We used PC1 scores from a principal component analysis of the variance–covariance matrix of tarsus, wing and weight (using residuals from annual means) as an index of body size. PC1 explained 45.2% of the variation and the respective character loadings for tarsus, wing and weight were 0.58, 0.58 and 0.57.
Throughout each breeding season (May–early July) we visited each territory every 1–4 days and marked on to smallscale maps the location of territorial boundaries (judged from male song perches and male–male chases), nests and any unbanded males or females we observed. We found nests by watching females building and by searching vegetation, and checked them every 2–4 days. We failed to locate fewer than 5% of nests, judging from parents’ behaviour at times we could not find a nest. For the minority of nests found after laying had ceased, we used clutch size, hatch date, average incubation period, and assumed that females laid daily, to determine laying dates. In 1992 we took blood samples of 10–50 μl from the jugular vein of nestlings that reached 5 days of age, although 61% of nests were depredated before reaching this stage. Therefore, to increase the proportion of nests sampled, in 1993 and 1994 we collected eggs (under permit EK0651 from the Canadian Wildlife Service) and used the embryos as a source of DNA for parentage analysis (see below).