66th Supplement to the Check-list of North American Birds publishes today in Ornithology

CHICAGO — August 14, 2025  — The 66th Supplement to the American Ornithological Society’s (AOS’s) Check-list of North American Birds, published today in Ornithology, includes several significant updates to the classification of bird species found in North America, Central America, and the Caribbean.

A few highlights from the supplement, detailed below, include species splits for Myiarchus nuttingi, Vireo gilvus, and Larus argentatus; the addition of subfamilies in the Laridae for white-terns and noddies; and a merging of three families of Caribbean nine-primaried oscines.

The Check-list, published since 1886, is updated in annual supplements from the AOS’s North American Classification Committee (NACC). The Check-list and its supplements provide the taxonomic and nomenclatural foundation for bird research, conservation, management, and education throughout the region, and are relied on as the authority on avian biodiversity by government agencies, NGOs, scientists, and birders, among others. The NACC reviews proposals submitted annually for taxonomic and distributional updates to the Check-list of North American Birds.

The full 2025 Check-list supplement is available at: https://doi.org/10.1093/ornithology/ukaf015. 

New Species, Genera, and Subfamilies

Until recently, it was thought that the Nutting’s Flycatcher (Myiarchus nuttingi) complex consisted of three subspecies: M. n. inquietus in western Mexico, and M. n. nuttingi and M. n. flavidior in Middle America. An analysis of hundreds of sound recordings of these subspecies, among other data, resulted in a species split and elevation to species status for M. flavidior, now called Salvadoran Flycatcher. In their proposal (2025-A-4), authors Roselvy Juárez, John van Dort, and Oscar Johnson stated, “The two taxa are sympatric in multiple locations with no sign of interbreeding, have diagnostic songs that are as different as those between other species of Myiarchus, and occupy different habitats across a broad swath of Central America.”

“The vocal differences were quantified nicely,” Chesser remarks. “and the fact that there’s no evidence of interbreeding where these species co-occur clinched the argument for species status.” 

Get ready to add a new vireo species to your checklist! NACC co-chair Carla Cicero submitted a proposal (2025-C-3) to elevate different subspecies of Vireo gilvus (Warbling Vireo) to species status. As it turns out, the Eastern (gilvus) subgroup and the Western (swainsoni) subgroups “are separate species based on differences in a suite of characters, including genetics, vocalizations, and molt and migration; importantly, these differences are maintained where they breed parapatrically,” says Chesser. These species breed assortatively where they meet, despite low levels of hybridization. Vireo swainsoni is now the Western Warbling-Vireo, and V. gilvus is now the Eastern Warbling-Vireo.

Despite some striking morphological similarities, hawks within the genus Accipiter have been found through new molecular evidence not to comprise a monophyletic group, as documented in a proposal (2025-C-1) from Shawn M. Billerman. He proposed a new generic arrangement of species within the family Accipitridae, including adding two new genera to the Check-list. Notably, this revision resulted in the transfer of Accipiter cooperii (Cooper’s Hawk) and several other hawks from Accipiter into the newly recognized genus Astur. This might be surprising to birders, considering the striking morphological similarities between A. cooperii, which is now in Astur, and A. striatus (Sharp-shinned Hawk), which will remain in Accipiter. “People have trouble telling Cooper’s and Sharp-shinned Hawks apart, but it turns out that they’re not that closely related,” Chesser remarks. “Raptors in general show a lot of convergence, and it turns out that the harriers are actually more closely related to species of Astur than Astur and Accipiter are to each other.” Other hawk species moved to Astur include A. gundlachi (Gundlach’s Hawk), A. bicolor (Bicolored Hawk), A. gentilis (Eurasian Goshawk), and A. atricapillus (American Goshawk), and A. soloensis (Chinese Sparrowhawk) has been transferred to the genus Tachyspiza.

The large, complex family Laridae has undergone some significant changes at several taxonomic levels as a result of two proposals (2025-C-2 and 2025-A-3). Within the long-confounding genus Larus, which includes the large white-headed gulls, L. argentatus (formerly Herring Gull) has been split into four species based on an array of genetic, phenotypic, ecological, and vocal differences: L. argentatus (now European Herring Gull), L. vegae (Vega Gull), and  L. smithsonianus (American Herring Gull) as well as the extralimital L. mongolicus. Chesser explains more about the challenges within this group, saying, “They are closely related, recently separated, and they also hybridize. That presents a confusing genetic signature, but they do not appear to be each other’s closest relatives, certainly not as a group.”

Also within the Laridae, the subfamily Sterninae has been significantly revised because some species have been shown to be more divergent from other terns than previously thought, based on a proposal (2025 A-3) by H. Douglas Pratt and Eric VanderWerf, with contributions from the late Storrs Olson. Notably, birds of the genera Gygis (white-terns) and Anous (noddies) have been moved from the Sterninae into their own new subfamilies, Gyginae and Anoinae, respectively.  “We thought the white-terns and noddies were closely related to each other and to other terns,” Chesser explains, “but they’re actually all quite distinct.” In addition, based largely on morphology, vocalizations, and, in one case, archeological evidence of historical sympatry, G. candida (Blue-billed White-Tern) and G. microrhyncha (Little White-Tern) have been separated as new species from G. alba (now called Atlantic White-Tern).

Family Mergers

Merger for three families of Caribbean nine-primaried oscines 

A proposal (2025-C-11) from Kevin J. Burns led to the merging of two families of Caribbean nine-primaried oscines into a third family, the Phaenicophilidae, which is now named the Greater Antillean Tanagers and comprises two subfamilies and nine species. Data from multiple molecular phylogenetic studies provided evidence that these species constitute a monophyletic group resulting from an endemic Caribbean radiation, facts highlighted by the new family classification. Species originally in the family Phaenicophilidae are also now placed in the subfamily Phaenicophilinae (Hispaniolan Tanagers) and those originally in the families Nesospingidae and Spindalidae now constitute the subfamily Spindalinae (Puerto Rican Tanagers and Spindalises).

About the journal

The journal Ornithology is a peer-reviewed, international journal of ornithology published by the American Ornithological Society (AOS). Ornithology (formerly The Auk) commenced publication in 1884 under the banner of the American Ornithologists’ Union, the AOS’s predecessor society. In 2009, Ornithology was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

About the American Ornithological Society

The American Ornithological Society (AOS) is an international society dedicated to connecting ornithologists, science, and bird conservation by supporting science that advances the understanding and conservation of birds; promoting broad access to ornithological science; supporting ornithologists throughout their career paths; and fostering a welcoming, diverse, supportive, and dynamic ornithological community. The AOS publishes two top-ranked international scientific journals, Ornithology and Ornithological Applications, and hosts an annual conference that attracts ornithologists from across the globe. Its robust grants program supports student and early-career professional research initiatives. The society’s check-lists serve as the accepted authorities for scientific nomenclature and English common names of birds in the Americas. The AOS is also a partner with The Cornell Lab of Ornithology in the online Birds of the World, a rich database of species accounts of the world’s birds. The AOS is a 501(c)(3) nonprofit organization serving about 3,000 members globally. For more information, see www.americanornithology.org.