Many bird species exhibit both social monogamy and genetic polygamy. Rates of genetic polygamy (often reported as the proportion of nests containing extra-pair young) in socially monogamous species vary widely, stimulating much research into the potentially adaptive value of these behaviors. Similarly, adult and nestling sex ratios of bird populations are not infrequently male-biased, leading to questions of the adaptive value of a female’s ability to influence sex ratios within her nest. Empirical data on both aspects of reproductive behavior, however, are still scarce for a majority of species even within relatively well-studied bird communities. We sampled DNA from nestling (n = 153) and adult (n = 121) Seaside Sparrows (Ammospiza maritima) at 58 nests in southeastern Louisiana from 2012 to 2017. We used microsatellite genotype analyses to discover that (1) ∼32% of nests contained broods with mixed paternity, and (2) ∼4% of broods contained evidence of mixed maternity. We sexed all birds via PCR and found a significant male bias among nestlings (62% of all nestlings were males). Prevalence of mixed paternity and male bias among nestlings did not appear to be related to site oiling status following the Deepwater Horizon oil spill, site context, timing within breeding season, or density of adult Seaside Sparrows or nests. Nonetheless, these results indicate a need for further work to understand the breeding biology of Seaside Sparrows.