Baltimore Oriole

Baltimore Oriole
In 1973, the American Ornithologists Union grouped the Baltimore Oriole together with the Bullock's (I. Bullockii) and called them "Northern Oriole." This was due to behavioral similarities and the fact that there is a zone of contact in the Great Plains where Baltimores interbreed with Bullock's. Further studies have determined that the two forms are deserving of full species status after all. In fact, recent molecular analyses have discovered that they are not even each other's closest relatives; the Baltimore Oriole is most closely related to the Black-backed Oriole of Mexico, according to one mitochondrial DNA study. The genetic relationships between oriole species are the subject of continuing research.

The westward range movement of Baltimore Orioles is considered to be the result of suitable habitat created as European settlers planted trees on their farms. With the advent of modern agriculture and the plowing of the prairies, Baltimore Orioles have continued to expand west along streams and rivers. In some areas, the Baltimore x Bullock's hybrid zone has shifted westward due to this expansion; in other areas, it is the Bullock's Oriole that seems to be moving eastward. In any event, the zone of hybridization has remained approximately the same width, and orioles within this zone may show intermediate plumage.
Baltimore Oriole Range Map

Upon return from their Central American wintering grounds in the spring, male Baltimore Orioles begin establishing territories and singing their familiar, loudly whistled song in open woodlands and forest edges, in isolated clumps of tree, and other open areas, often in suburban settings. Individual males tend to sing a unique song. Competing males may countersing, sometimes imitating each other's calls. Males, which precede the arrival of females by two or three days, display to prospective mates with repeated exaggerated bowing, showing alternately their orange belly and then their black back and orange rump. Once a pair is established, both the male and female defend their relatively small territory. The pair maintains contact through calls and song.

The nest is typically suspended from the end of a branch near the top of a tree. It is a woven sack, as deep as 8 inches, with an opening at the top approximately 3 ½ inches in diameter. Females are the primary architects of the nest, incorporating into the construction the fibers of grapevine, grasses, dogbane, milkweed, and, where available, Spanish moss. When horses were more common, many observers noted the prevalence of long horsehairs in oriole nests. Frequently, birds return to the same territory in subsequent years, though they seldom reuse old nests.

The male Baltimore is the only oriole with a fully black hood and back and orange in the tail. The rump and underparts are orange. Adult males have a single white wingbar and an orange shoulder patch on black wings. Adult female and first-summer male are quite variable, showing two white wingbars, various amounts of orange in the underparts, and black on throat, hood or upperparts. The presence of orange differentiates female Baltimore Orioles from the similar but yellowish Orchard Oriole (I. Spurius).

Like the male, the female Baltimore Oriole resemble female Bullock's Orioles but have more extensive orange or orangish-yellow on the underparts, which fades gradually to the gray of the belly. Bullock's females show an abrupt change from a yellowish throat to the gray belly and have more extensive yellow on the cheeks and a yellowish supercilium contrasting with darker eyeline. Baltimore Orioles have a yellowish wash to rump while the rumps of Bullock's are grayish.

In spring, male whistles short, individualized flute-like songs almost constantly until mated. By mid-May, most birds still singing continuously are likely unpaired first-summer birds, since few are successful in gaining mates. Songs heard late in the season are usually those of unmated or immature birds.

Attracting Orioles