Unexpected Alliances: Same-Sex Parenting in the Black-Vented Shearwater

Same-sex parenting is known in many species, including seabirds, but is rarely reported in burrow-nesting seabirds. This study shares the first documented case of two female Black-vented Shearwaters Puffinus opisthomelas forming a pair, observed on Natividad Island, Mexico, where most of the population breeds. Our findings reveal that two females have co-occupied the same burrow, successfully raising three chicks in 2017, 2019 and 2023. Although it is unclear exactly how the eggs were fertilized, this case demonstrates that same-sex pairs can be as reproductively successful as male-female pairs, suggesting a cooperative breeding strategy. The stability of this pairing and their reproductive success point to the possibility that same-sex pairs might play a more significant role in the breeding dynamics of Black-vented Shearwater. We believe this record adds to the evidence of the evolutionary significance of same-sex behaviours in seabirds, offering new insights into the reproductive dynamic of this species.