Little geographic variation in plumage or size among North American populations. Caribbean breeders (ridgwayi) noticeably paler on crown and breast than their North American counterparts, appearing almost white-headed and white-breasted and showing little difference between male and female. Occasionally paler individuals, like Caribbean breeders, found breeding in south Florida, where most individuals similar to breeders farther north; these observations suggest, perhaps not surprisingly, possibility of limited interchange between Caribbean and North American populations. In Baja California, Mexico, residents are much like Caribbean birds, with breasts and crowns almost entirely white, and also with white underwing coverts; females slightly but not significantly darker than males in both these populations, so sexes not reliably distinguished (Blanco and Rodriguez-Estrella 1999).
Four subspecies worldwide recognized following Prevost 1983b (as modified in Marchant and Higgins 1993; see below). Australian birds considered most distinctive among all taxa (Prevost 1983b); otherwise quite similar in size and plumage. Identification to subspecies requires birds in Definitive plumage.
P. h. carolinensis: Breeds in North America, north of Caribbean (see Distribution); winters to South America.
P. h. ridgwayi: Resident in Caribbean from Bahamas and Cuba south to coast of se. Mexico and Belize; not currently found breeding south of southern Belize and Cuba. Underwing primary coverts mostly white with pale brown restricted to distal two-thirds of outer web; head largely white (dark markings reduced on crown and sides of head); breast markings reduced or lacking on both sexes; relatively small (averaging smaller than carolinensis). Often paler brown dorsally (perhaps sun bleaching).
P. h. haliaetus: Breeds in Palearctic (Europe, northwest coast of Africa, and Asia north of the Himalayas); winters to southern Africa, India, and East Indies.
P. h. leucocephalus: Name P. h. cristatus formerly used for this subspecies. Resident breeder in Australia and southwest Pacific (New Guinea west to Celebes and perhaps Java, east and south to New Caledonia and Vanuatu; see details in Poole 1989a and Marchant and Higgins 1993).