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). Check-list of North American Birds. 6th edition. American Ornithologists' Union, Washington, DC, USA. Close 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, J. D. (1996). The stability of the oriole hybrid zone in western Kansas. Condor 98:658-663. Close Rising 1996c, Carling, M. D., L. G. Serene, and I. J. Lovette (2011). Using historical DNA to characterize hybridization between Baltimore Orioles (Icterus galbula) and Bullock’s Orioles (I. bullockii). Auk 128:61-68. Close Carling et al. 2011), and in 1995 the 2 were once again recognized as separate species ( American Ornithologists' Union (1995). Fortieth supplement to the American Ornithologists' Union Check-list of North American Birds. Auk 112:819-830. Close 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, B. C., and R. F. Mearns (1992). Audubon to Xántus: The lives of those commemorated in North American bird names. Academic Press, New York, NY, USA. Close 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, P. L. (1982d). A comparison of colonial and non-colonial nesting by Northern Orioles in central coastal California. Master's Thesis, Univ. of California, Berkeley. Close Williams 1982d, Williams, P. L. (1988d). Spacing behavior and related features of social organization in Northern Orioles of central coastal California. Ph.D. thesis, Univ. of California, Berkeley. Close Williams 1988d, Richardson, D. (1997). Breeding density, male age and mixed reproductive strategies in the Northern Oriole (Icterus galbula bullockii), University of Leicester, Leicester. Close Richardson 1997), in California and Washington ( Butcher, G. S. (1984a). Sexual color dimorphism in orioles (the genus Icterus): tests of communication hypotheses. Ph.D. thesis, Univ. of Washington, Seattle. Close Butcher 1984a), in California, Nevada, Utah, and Oklahoma ( Pleasants, B. Y. (1979). Adaptive significance of the variable dispersion pattern of breeding Northern Orioles. Condor 81:28-34. Close Pleasants 1979), and in Crook, Colorado, where Bullock’s and Baltimore orioles breed sympatrically and limited hybridization may be occurring ( Edinger, B. B. (1985). Limited hybridization and behavioral differences among sympatric Baltimore and Bullock's orioles. M.S. thesis, University of Minnesota, Minneapolis, MN, USA. Close Edinger 1985). The molt sequence and location of molt has also been examined ( Rohwer, S. and J. Manning (1990). Differences in timing and number of molts for Baltimore and Bullock's orioles: Implications to hybrid fitness and theories of delayed plumage maturation. Condor 92:125-140. Close Rohwer and Manning 1990, Rohwer, S., and M. S. Johnson (1992). Scheduling differences of molt and migration for Baltimore and Bullock's orioles persist in a common environment. Condor 94:992-994. Close Rohwer and Johnson 1992, Rohwer, V. G., S. Rohwer and M. F. Ortiz-Ramirez. (2009c). Molt biology of resident and migrant birds of the monsoon region of west Mexico. Ornitología Neotropical 20 (4):565-584. Close Rohwer et al. 2009c, Pillar, A. G., P. P. Marra, N. J. Flood and M. W. Reudink. (2016). Moult migration in Bullock's Orioles (Icterus bullockii) confirmed by geolocators and stable isotope analysis. Journal of Ornithology 157 (1):265-275. Close 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, J. D. (1973). Morphological variation and status of the orioles, Icterus galbula, I. bullockii, and I. abeillei, in the northern Great Plains and in Durango, Mexico. Canadian Journal of Zoology 51:1267-1273. Close Rising 1973b, Rising, J. D. (1996). The stability of the oriole hybrid zone in western Kansas. Condor 98:658-663. Close Rising 1996c, Carling, M. D., L. G. Serene, and I. J. Lovette (2011). Using historical DNA to characterize hybridization between Baltimore Orioles (Icterus galbula) and Bullock’s Orioles (I. bullockii). Auk 128:61-68. Close Carling et al. 2011, Jacobsen, F., and K. E. Omland (2012). Extensive introgressive hybridization within the Northern Oriole group (genus Icterus) revealed by three-species isolation with migration analysis. Ecology and Evolution 2:2413-2429. Close Jacobsen and Omland 2012). Much, however, remains to be learned about the Bullock’s Oriole (see Priorities for Future Research).