Phylogeography and species limits in the red‐shouldered hawk (Buteo lineatus): Characterization of the Northern Florida Suture Zone in birds

The North American red‐shouldered hawk, Buteo lineatus, is comprised of two widely allopatric eastern and western populations with an additional well‐marked subspecies in the Florida peninsula. The two eastern populations meet in northern Florida, the location of a well‐known suture zone in many nonavian organisms. We sequenced the complete mitochondrial ND2 gene and two nuclear introns to investigate its genetic population structure and species status. No mitochondrial haplotypes were shared between the eastern and western populations, and genetic variance among 14 populations was 0.42; almost all of this (0.40) was distributed among the three regions. A clade of haplotypes very common in the Florida peninsula decreased in frequency elsewhere and, when modeled as a hybrid zone, had an estimated width of 1,158 km with a center near Ocala, FL. Ecological niche modeling suggests the western, eastern, and Florida peninsula populations were geographically isolated during the last glacial maximum. We consider these to represent three phylogenetic species. A coalescent analysis incorporating incomplete lineage sorting and gene tree uncertainty also suggested the divergence between the western and eastern populations is consistent with species‐level divergence. With the addition of this hawk, four avian species are now known to hybridize along the Gulf Coast of the United States in or near the Northern Florida Suture Zone. The widths of these avian zones vary substantially (176–1,158 km) and appear to reflect magnitude of gene flow, rather than extent of genetic differentiation. None of these birds was suggested as possible exemplars in the original description of the suture zone. Of the six species that were so identified, three have been surveyed to date, but none of those was found to be genetically differentiated. DNA sequences indicate the Buteo lineatus complex is composed of three species‐level taxa, two of which hybridize in a well‐known suture zone in northern Florida, USA. Three additional species pairs of birds hybridize in the same region. The four hybrid zones vary in width and position; their widths appear to scale with dispersal distances. (Photo: courtesy Dolores Albertuzzi).