Extrapair paternity in a far northern population of Yellow Warblers Dendroica petechia

The proportion of extrapair paternity is known to vary greatly among species, but differences between populations of the same species have rarely been considered. We used microsatellite DNA markers to assess parentage of offspring in a subarctic population of the socially monogamous Yellow Warbler Dendroica petechia nesting near Churchill, Manitoba. We found a significantly lower proportion of extrapair young in the Churchill population than in a temperate population nesting approximately 1500 km to the south near the Queen's University Biological Station (QUBS) in Ontario. We show that the Churchill population also had significantly lower nesting density and significantly higher breeding synchrony; both are factors that have been hypothesized to affect extrapair paternity negatively. We suggest that inter-population comparisons can be used to test proximate mechanisms affecting extrapair paternity and in some cases may be better than interspecific or inter-individual comparisons. Towards this end we list eight other species that have had extrapair paternity measured in different populations. Studies that use genetic markers to assess the parentage of offspring have shown that extrapair paternity occurs in a wide range of bird species and that there is considerable interspecific variation in the proportion of offspring fathered by extrapair males (see Westneat and Sherman 1997). Understanding the variation in extrapair paternity has become a focus of mating system research. Nearly all species studied to date, however, have had extrapair paternity measured in only one population. Thus, it is not known how much of the apparent interspecific variation in extrapair paternity might be due to variation between populations within a species. For example, in the Willow Warbler Phylloscopus trochilus, the proportion of extrapair paternity differs considerably between a population in Sweden (0%; Gyllensten et al. 1990) and one in Norway (33%; Bjornstad and Lifjeld 1997). In this study we use microsatellite DNA markers to assess the parentage of offspring in a far northern population of the socially monogamous Yellow Warbler Dendroica petechia. Our first aim was to test whether the proportion of extrapair paternity in this population at the northern edge of the species' range differs from the 33% extrapair paternity in a population 1500 km to the south (Yezerinac et al. 1995). Nesting density and breeding synchrony are the most likely proximate causes of variation in the proportion of extrapair paternity (Westneat and Sherman 1997). However, evidence is mixed regarding the effect of either factor. Between-species comparisons provide no evidence that density affects extrapair paternity, whereas comparison of individuals nesting at different densities in the same population does suggest that density sometimes positively affects extrapair paternity (see Westneat and Sherman 1997). The apparent effect of synchrony on extrapair paternity is also debatable based upon results of interspecific and intraspecific studies (see Westneat and Sherman 1997, Stutchbury 1998, Weatherhead and Yezerinac 1998). In this study we measured nesting density and breeding synchrony within each of the two populations for which extrapair paternity was measured. Our second aim was to determine if there were synchrony and density differences between the populations that might explain any differences in extrapair paternity.