A characteristic bird of open woodland in western North America—especially riparian woodlands with large cottonwoods (Populus spp.), sycamores (Platanus spp.), and willows (Salix spp.)—Bullock’s Oriole overwinters in western Mexico. In summer, this species eats mostly arthropods, readily augmenting its diet with ripe fruit. Older males have bright black and orange-yellow plumage and females have a generally grayer body with yellower head, breast, and tail; younger males resemble females, but have black on the throat.
Bullock’s Oriole hybridizes frequently with the Baltimore Oriole (Icterus galbula) in the Great Plains region at the eastern limit of its distribution, even though these 2 species differ markedly in appearance, behavior, molt cycles, and vocalizations, and somewhat in size. Because of this hybridization, these 2 species were at one time considered a single species, the Northern Oriole (I. galbula; American Ornithologists' Union 1983 ). Most of the interbreeding occurs in the Great Plains, however, even in areas where hybrids are frequent, many individuals are of the parental phenotypes. In addition, the width of the hybrid zone appears to be stable ( Rising 1996c , Carling et al. 2011 ), and in 1995 the 2 were once again recognized as separate species ( American Ornithologists' Union 1995 ).
Bullock’s Oriole was described and named by William Swainson in 1827 on the basis of material collected by William Bullock and his son, also William. In his description, Swainson wrote, “This, the most beautiful of the group yet discovered in Mexico, will record the name of those ornithologists who have thrown so much light on the birds of that country” ( Mearns and Mearns 1992a : 555).
Bullock’s Oriole is less well studied than its eastern counterpart, the Baltimore Oriole. Aspects of the behavior, ecology and social organization of Bullock’s Oriole have been studied in California ( Williams 1982d , Williams 1988d , Richardson 1997 ), in California and Washington ( Butcher 1984a ), in California, Nevada, Utah, and Oklahoma ( Pleasants 1979 ), and in Crook, Colorado, where Bullock’s and Baltimore orioles breed sympatrically and limited hybridization may be occurring ( Edinger 1985 ). The molt sequence and location of molt has also been examined ( Rohwer and Manning 1990 , Rohwer and Johnson 1992 , Rohwer et al. 2009c , Pillar et al. 2016 ). Hybridization between Bullock’s and Baltimore orioles in the Great Plains, and between Bullock’s and Black-backed (I. abeillei) orioles in Mexico have been well studied (e.g., Rising 1973b , Rising 1996c , Carling et al. 2011 , Jacobsen and Omland 2012 ). Much, however, remains to be learned about the Bullock’s Oriole (see Priorities for Future Research).