Bullock's orioles are sexually dimorphic, with males being more brightly colored than females. In addition, adult males tend to be slightly larger and heavier than females. In general, adults range in mass from. Adults have a pointed bill with a straight culmen. In adult males, the tail is long, square, and jet black. All exposed skin is black, as are the claws and bill, though the base of the lower mandible lightens to bluish-gray. Adult males are characterized by strongly contrasting orange and black plumage, a black throat patch and a white wing bar. The underparts, breast, and face are orange or yellow; by contrast, the back, wings, and tail are black. A black line extends from each eye to the black crown. The wing coverts are fringed white, forming a wing patch. Although the tail is mostly black, the outermost three or four rectrices are tipped orange, forming a T shape. Adult females, by contrast, have gray-brown upperparts, duller yellow on the breast and underparts, and an olive crown. Some females may also have a dark throat patch, similar to the one found in adult males; in all cases females lack the black eye-line present in adult males. It is hypothesized that females with throat patches are older individuals. Following the general pattern observed among icterids, the overall plumage pattern seen in immature male Bullock's orioles closely resembles that seen in adult females. Juveniles resemble adult females but have darker wings, fresh wing coverts, and a pink or whitish bill. Sexual dimorphism is not obvious in juveniles.
Reproduction
Bullock's orioles are seasonally monogamous. The breeding season typically lasts from May until July. The exact timing of the beginning of the breeding season tends to vary geographically: in general, breeding begins later in the northernmost and westernmost portions of its geographic range. Mated pairs of Bullock's orioles cooperate to weave deep, pendant baskets in which are deposited between three and six eggs, though females tend to do much of the work. The nest is woven of plant fibers, primarily bark and fine grass fiber, though animal hair is also commonly used. The nest is lined with down, hair, and moss. Both males and females rear the young and defend the nest from predators and nest parasites. Bullock's orioles and Baltimore orioles typically hybridize in the Midwest where their geographic ranges overlap.
Communication
Both males and females sing. While males have a sweeter voice, females tend to be more prolific singers. This bird's song is similar to that of the Baltimore oriole, but faster and somewhat more harsh.
Distribution
Bullock's orioles are native to western North America, though according to Jaramillo they are sometimes found as vagrants in the eastern half of the continent. During the breeding season, they are found as far west as the eastern foothills of the Cascade range. Their breeding range stretches east to the Dakotas, Kansas, and northern central Texas. This species can be found as far north as British Columbia in Canada and as far south as Sonora or Durango in Mexico. It is common throughout its range, but is absent in parts of Arizona and Idaho where a combination of extreme elevation and an arid climate make for poor living conditions. During winter, this species retreats to Mexico and northern Central America. Its winter range extends south and east from Sinaloa to Oaxaca.
Behavior
Habitat
Like other members of Icteridae, Bullock's orioles prefer habitat edges. They especially prefer riparian corridors, open deciduous woodland, and scrub forest. Observations during the breeding season indicate that members of this species prefer areas with an abundance of cottonwood, pecan, and willow. In dry areas, this species prefers salt cedar and mesquite. In California, eucalyptus trees are used as major sources of nectar.
Diet
These birds forage in trees and shrubs, also making short flights to catch insects. They mainly eat insects, berries, and nectar. Other fruits eaten include oranges and sometimes grapes. In some locations, they may be seen using hummingbird feeders. These birds will readily come to a dish filled with grape jelly. Their parents lead the fledglings, feed them jelly and then the adult males leave the area. The young stay at the jelly dish from first feeding in early July and leave late September.
Related species
A member of the northern oriole group, Bullock's orioles were once considered to be conspecific with Baltimore orioles and black-backed orioles. However, recent phylogenetic data suggest that Bullock's orioles are members of a distinct species that does not share a most-recent common ancestor with Baltimore orioles, as was once assumed.
