A characteristic bird of open woodland in western North America—especially riparian woodlands with large cottonwoods, sycamores, and willows—Bullock's Oriole winters 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, younger males resemble females, which are green and yellow, with black on the throat.
Bullock's Oriole hybridizes frequently with the eastern Baltimore Oriole (Icterus galbula) in the Great Plains region, although these 2 species differ markedly in their appearance, behavior, molt cycles, and vocalizations, and somewhat in size. Because of this hybridization, these 2 species have been considered a single species, the Northern Oriole (I. galbula; American Ornithologists' Union 1983 ). Most of the interbreeding occurs in the Great Plains, however, and even in areas where hybrids are frequent, many individuals are of the parental phenotypes. In addition, the width of the hybrid zone appears not to be increasing ( Rising 1996c ), and recently these 2 have again been 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. The breeding behavior and social organization of Bullock's Oriole have been studied in California (Williams Williams 1982d , Williams 1988d ), as has molt sequence ( Rohwer and Manning 1990 , Rohwer and Johnson 1992 ). 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 (Rising Rising 1973b , Rising 1996c , and references therein). Much, however, remains to be learned about Bullock's Oriole (see Priorities for future research, below).