Abaunza G., López-Hernández J.A., Soldatini C. & Albores-Barajas Y.V. (2025) Unexpected alliances: Same-sex parenting in the Black-vented Shearwater. ARDEA 113 (1): 134-138

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 samesex 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.