A seasonal shift in offspring sex ratio of the brood parasitic Brown-headed Cowbird (Molothrus ater)

Avian obligate brood parasites do not provide parental care for their eggs and young, and may therefore serve as a strong model system to test predictions of evolutionary sex-allocation theories, independent of parental modulation of primary sex ratios. However, none of the handful of previous studies examining offspring sex ratio in brood parasitic birds have revealed a bias from parity at the level of the female parasite, the host species, or temporal scale(s). This is also surprising, because in at least one brood parasite, the Brown-headed Cowbird (Molothrus ater), adult sex ratios are consistently and heavily male-biased. Here we used a large database of embryonic and nestling cowbirds’ genetic sex ratios collected from nests of a single host species, the Prothonotary Warbler (Protonotaria citrea) to assess potential overall, temporal, and individual patterns of bias. Contrary to previous findings, we documented an increase in the calculated male sex ratios later in the breeding se...