Extrapair paternity is influenced by breeding synchrony and density in thecommon yellowthroat

The effects of breeding synchrony and density on levels of extrapair patern ity in birds are controversial. We used multilocus DNA fingerprinting and m icrosatellite analysis to examine the effects of breeding synchrony and den sity on levels of extrapair paternity in the common yellowthroat (Geothlypi s trichas). As in many Neotropical migrants, breeding synchrony was greates t at the beginning of the breeding season. Levels of extrapair paternity we re higher after the peak in synchrony, leading to an overall negative relat ionship between extrapair paternity and breeding synchrony. However, there was a significant interaction between breeding synchrony and density, as le vels of extrapair paternity were higher only for males breeding when both s ynchrony and density were low. We discuss several possible explanations for this interaction, including lower quality males or territories in low dens ity areas and greater demands on mate guarding among males with larger terr itories. Most studies have not considered simultaneously the effects of bre eding synchrony and density on extrapair paternity. Our results suggest tha t ecological correlates of paternity may be revealed only after testing for interactions in multivariate analyses.