2026/06/10 by Eamon C Corbett, Robb T Brumfield, Brant C Faircloth
paper · doi:10.1093/ornithology/ukag028
Abstract The large grackles in the subgenus Cassidix—Quiscalus mexicanus (Great-tailed Grackle), Q. major (Boat-tailed Grackle), and Q. palustris (Slender-billed Grackle)—form a clade with a complex evolutionary history of recent divergence and secondary contact. Quiscalus mexicanus is widespread from the western U.S. to Peru and encompasses 2 divergent mitochondrial lineages. Quiscalus major is found on the Atlantic and Gulf Coasts of the U.S. and shows striking geographic variation in eye color. Quiscalus palustris was endemic to central Mexico but went extinct during the early 20th century. Previous research has called into question the monophyly of Q. mexicanus and has suggested that Q. major and Q. mexicanus may hybridize in Texas and Louisiana. Additionally, the patterns of genetic structuring among subspecies within Q. mexicanus and Q. major are unclear. We resolved many of these questions by sequencing and analyzing data from nuclear ultraconserved elements (UCEs) and mitochondrial DNA collected from samples representing all 13 subspecies in the Cassidix clade. Our results show no evidence of admixture between Q. major and Q. mexicanus, suggesting that if hybrids occur in the contact zone, they are infertile or rare. Our results also support the monophyly of Q. major and Q. mexicanus, with Q. palustris likely sister to a unified Q. mexicanus, contrary to evidence from mitochondrial DNA. Finally, these data resolve intraspecific relationships in the Cassidix complex, several of which differ from current subspecies-level taxonomy, and show that geographic breaks, not eye color differences, structure genetic variation within Q. major.