Bird of the Month: Bullock's Oriole
By Andy McCormick
Scientific Name: Icterus bullockii
Length 9 in
Wingspan 12 in
Weight 1.3 oz
AOU Band code BUOR
Anyone who has grown up in the eastern United States knows about the Baltimore Oriole as a woodland and garden bird and the mascot of the baseball team with the same name. Less well known, however, is Bullock’s Oriole, the western North American counterpart to the Baltimore Oriole Icterus galbula. Both share the genus Icterus, from the Greek ikteros, jaundice, and the common name from the Latin oriolus, golden (Holloway). The species name bullockii, was named in honor of William Bullock (1775-1855) by William Swainson. Bullock was a mine owner and bird collector and supported Swainson’s birding expeditions and artwork. The orioles share the same Family Icteridae with the Bobolink, blackbirds, grackles, meadowlarks and cowbirds and are the most colorful of that group.
After a brief stint being grouped with the Baltimore Oriole as the Northern Oriole, Bullock’s Oriole was separated again in 1998 and is considered a species on its own once more. There is, nonetheless, a narrow zone in the Great Plains where the two species hybridize. Identifying females of these species can be very difficult in this zone.
The male Bullock’s Oriole has three distinct field marks which are diagnostic: a bright orange supercilium (“eyebrow” stripe), a black eye line on an otherwise orange face, and a solid white wing patch. The Baltimore Oriole has a solid black head and back and a thin white wing bar. The female Bullock’s has an orange-yellow head but shows a ghost pattern of the male with brighter yellow supercilium and a dark eye line (Alderfer).
The Bullock’s Oriole is not as well-studied as its eastern counterpart, but is thought to have similar breeding behavior. It prefers riparian woodlands and in the northwest is particularly fond of nesting in cottonwoods. During migration orioles can be found in a wide variety of open wooded habitats. The nest is a hanging sack tightly woven with hair, grass, vines, plant fibers and sometimes yarn and string. It is attached to a thin branch in a tree often near water. Three to five pale blueish or greyish white eggs are deposited. Incubation lasts about two weeks and the young fledge in another two weeks (Rising and Williams).
Bullock’s Orioles like other orioles start their southerly migration early moving in July and August to the Sinaloa area of Mexico. They are one of a group of birds known as “molt migrants” who leave their nesting area early and stop in the Sinaloa thorn forests for nourishment and a fall molt after migrating part way. However, research in Sinaloa by Seivert Rohwer of the University of Washington found Hooded and Orchard Orioles with eggs in nests there (Powell). It is possible that some Bullock’s Orioles are also double breeders. More research is needed on this newly discovered phenomenon.