Data from: Extra-pair paternity correlates with genetic diversity, but not breeding density, in a Neotropical passerine, the Black Catbird

The frequency of extra-pair paternity (EPP) varies widely across socially
monogamous birds, but the proximate mechanisms driving this variation
remain unclear. In this study, we tested two major factors hypothesized to
influence extra-pair mating—breeding density and genetic diversity—by
comparing genetic mating patterns in two populations of black catbirds
Melanoptila glabrirostris. This Neotropical songbird is endemic to the
Yucatán Peninsula, including eastern Mexico, and its offshore islands. We
sampled one mainland (Sian Ka’an Biosphere Reserve) and one island (Isla
Cozumel) population and used single-nucleotide polymorphisms (SNPs) to
quantify heterozygosity and genetic parentage over two breeding seasons.
Moderate levels of EPP occurred in both populations (9.5 – 35% of
offspring and 17 – 45% of nests). Contrary to predictions, breeding
density did not affect EPP: although breeding densities were much higher
on the mainland than on the island, EPP rates did not differ between
populations, and local breeding density was not correlated with EPP at
individual nests. In contrast, partial support emerged for the hypothesis
that genetic diversity influences EPP: extra-pair offspring were more
heterozygous than within-pair offspring. However, the two populations did
not differ in genetic diversity, and neither the heterozygosity of social
fathers nor within-pair relatedness predicted EPP. These results are
consistent with recent comparative studies suggesting that breeding
density is not a critical driver of EPP rates, and that not all tropical
songbirds exhibit low rates of EPP.