Taxonomic status of Bay-winged Hawk Parabuteo (unicinctus) unicinctus and Harris's Hawk P. (u.) harrisi, with documentation of delayed plumage maturation in Bay-winged Hawk

Summary. The two main populations of Parabuteo unicinctus have long been treated as subspecies of the same species: Harris's Hawk P. u. harrisi in the southern USA to Costa Rica, and Bay-winged Hawk P. u. unicinctus in South America. However, they differ considerably in their morphology, number of plumages, and behavioural ecology. Adult Harris's Hawk differs in multiple plumage characters from adult Bay-winged Hawk, and differences are even more marked in juvenile plumage. Harris's Hawk has two age-related plumages but Bay-winged Hawk shows delayed plumage maturation and has four such plumages. Harris's Hawk breeds and hunts cooperatively, whereas Bay-winged Hawk nests only in pairs, and hunts individually. There are no valid records of Harris's Hawk in South America. We believe that the differences in adult and juvenile plumages, the number of immature plumages, and differences in breeding and hunting mean that Harris's Hawk and Bay-winged Hawk are best treated as separate species.