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. Methods Please see the README document ("README_Dataset-BlackCatbirdPaternity_v0.1.0.txt") and the accompanying published article: LaPergola, J.B., C. Riehl, J.E. Martínez-Gómez, B. Roldán-Clarà, and R.L. Curry. 2022. Extra-pair paternity correlates with genetic diversity, but not breeding density, in a Neotropical passerine, the Black Catbird. Journal of Avian Biology. Accepted. DOI: 10.1111/jav.03034