Nest sharing by birds, or the phenomenon where multiple individuals of different species contribute genetically and parentally to a single nest, is a rare form of cooperative breeding that has only occasionally been reported in socially monogamous birds. Here we describe, both behaviorally and genetically, the unique case of two female birds, a western kingbird (Tyrannus verticalis) and a western kingbird x scissor-tailed flycatcher (T. forficatus) hybrid simultaneously occupying (and likely co-incubating) a brood in a single nest. Both females provisioned nestlings, and they did this in two consecutive years (producing four fledglings each year). Genomic data from the females revealed that they were unrelated and parentage analyses revealed that both females contributed genetically to at least one of the offspring, and at least two fathers were involved. In total, this is the first reported case of nest sharing involving a hybrid individual and the first case within the family Tyrannidae.

New World wood warblers (family: Parulidae) can exhibit strong phenotypic differences among species, particularly in song and plumage. However, within-species variation in these warblers—often designated as subspecies—is much more subtle and has led to significant debate over the origin, maintenance, and conservation status of populations that differ. A species that exhibits controversial subspecific status is the black-throated green warbler (Setophaga virens), a Neotropical-Nearctic migrant that breeds throughout eastern and boreal North America with several isolated populations at the margins of its range. In particular, uncertainty has lingered over the status of S. v. waynei, a disjunct population along the southeast Atlantic Coastal Plain of the United States that differs morphologically and ecologically from the nominate subspecies. Despite its unique circumstances, the subspecific status of S. v. waynei remains questionable in the absence of any population-wide genomic analyses. Here, we employ whole-genome resequencing to estimate the genetic distinctiveness among samples collected across the entirety of S. virens breeding range, including from putative S. v. waynei. Despite detecting low global differentiation (FST = 0.027) across the entire species, we observed discrete genetic clustering among S. v. waynei. Principal components analysis of genome-wide differences shows the main axis of variation separates S. v. waynei from all other S. v. virens samples. We also found that S. v. waynei is most similar to another isolated population from the Piedmont of North Carolina and detected evidence of a historical north-to-south geographic dispersal among the entire complex. Combined with previously documented ecological and morphological distinctness, our results support that S. v. waynei be considered a distinct and recognized subspecies worthy of targeted conservation efforts.