Extrapair paternity is influenced by breeding synchrony and density in the common yellowthroat

The effects of breeding synchrony and density on levels of extrapairpaternity in birds are controversial. We used multilocus DNAfingerprinting and microsatellite analysis to examine the effectsof breeding synchrony and density on levels of extrapair paternityin the common yellowthroat (Geothlypis trichas). As in manyNeotropical migrants, breeding synchrony was greatest at thebeginning of the breeding season. Levels of extrapair paternitywere higher after the peak in synchrony, leading to an overallnegative relationship between extrapair paternity and breedingsynchrony. However, there was a significant interaction betweenbreeding synchrony and density, as levels of extrapair paternitywere higher only for males breeding when both synchrony anddensity were low. We discuss several possible explanationsfor this interaction, including lower quality males or territoriesin low density areas and greater demands on mate guarding amongmales with larger territories. Most studies have not consideredsimultaneously the effects of breeding synchrony and densityon extrapair paternity. Our results suggest that ecologicalcorrelates of paternity may be revealed only after testingfor interactions in multivariate analyses.