Extra-pair offspring are less heterozygous than within-pair offspring in American redstarts (Setophaga ruticilla)

The vast majority of bird species are socially monogamous; however, extra-pair paternity is nearly ubiquitous and a number of theories have been proposed to explain the prevalence of this mixed mating strategy. Here, we test the genetic compatibility hypothesis—the idea that females that are genetically similar to their social partners will mate with extra-pair males that are genetically dissimilar to produce offspring that are more heterozygous. For this study, we examined eight years of paternity data (2004-2011) from a Nearctic-Neotropical migratory bird, the American Redstart (Setophaga ruticilla), breeding in southeastern Ontario, Canada. We predicted that females paired with genetically similar males (higher relatedness) would be more likely to produce extra-pair offspring and that extra-pair offspring would have higher levels of heterozygosity than within-pair offspring. Alternatively, because this population experiences high levels of immigration, females may produce extra-pair...