Icterus bullockii

Bullock’s Oriole is a common breeding bird in western and southern Texas, where its hanging nests decorate large trees. The striking orange, black and white plumage of the adult male distinguishs it from the other Texas orioles and especially from the black and chestnut Orchard Oriole (Icterus spurius), the other widespread breeding oriole in Texas.

The relationship between Bullock’s and Baltimore (I. galbula) orioles has long puzzled ornithologists. Although they differ in appearance, behavior, molt cycles and vocalization, they hybridize freely on the Great Plains (north of Texas). Based on this hybrid data, the 2 species were considered conspecific (Northern Oriole, Am. Ornithol. Union 1983), but further study has shown that the two species are not each other’s closest relatives and the hybrid zone is stable, so they are again considered separate species.

Male Bullock’s Orioles do not acquire “adult” plumage until September-November of the second year. In an eight-year study in California the percentage of second-year males in a breeding population varied from 6 to 55% (average 25%, Pyle 1997, Rising and Williams 1999).

 

DISTRIBUTION. During the 1987-1992 field work period of the TBBA project Bullock’s Oriole bred mostly in the area between the 99th and 104th meridians, extending east to the 97th meridian south of the 30th parallel.

Elsewhere this species breeds through most of the western United States, northeastern Mexico and southwestern Canada. Highest breeding densities are found in California, western Texas, eastern New Mexico and southeastern Colorado. Bullock’s Orioles winter in west Mexico (Rising and Williams 1999, Sauer et al. 2003).

SEASONAL OCCURRENCE. Bullock’s Orioles migrate to and through Texas from late March to mid-May and breed (egg dates) from April 7 to July 5. Dates for 37 TBBA confirmed breeding observations ranged from May 3 to July 31. Southward migration starts in late July with most individuals gone by late September (extreme date November 22, Oberholser 1974).

BREEDING HABITAT. In Texas, Bullock’s Oriole is found in open woodlands where mesquite predominates, between sea level and 1725 m (5700 ft). In riparian areas, the species nests in willows and cottonwoods. It adapts to pecan trees in orchards. This oriole also adapts to human-created habitats, including irrigated fields, ranches, parks, and street trees (Oberholser 1974).

Nest construction starts after pairs arrive on the breeding grounds and takes up to 15 days. The nest, a woven, usually hanging, pouch, is often attached near the end of a branch (averaging about 8-9 m [25-30 ft] above ground). The nest, built by the female, is constructed of soft vegetable fibers (grasses, leaves, shreds of wild flax or bark) and lined with plant down, animal hair, or fine grasses. It is often placed in the same tree used by an Eastern (Tyrannus tyrannus) or Western (T. verticalis) kingbird. The nest is more pendulous than the nest of Orchard Oriole, but less pendulous than the nest of Baltimore Oriole. An Orchard Oriole nest also differs in being made primarily of grass (Oberholser 1974, Harrison 1979, Rising and Williams 1995).

The female Bullock’s Oriole incubates a clutch of 4-5 eggs (range 2-6) eggs for 11 days and nestlings leave the nest 14 days after hatching. A second successful brood in the same season is rare. During the breeding season the species eats mostly insects and other arthropods augmented by small fruits (Oberholser 1974, Rising and Williams 1999).

STATUS. In western Texas Bullock’s Oriole is a common summer resident (Lockwood and Freeman 2004). In much of its summer range in Texas, Breeding Bird Survey (BBS) observers found an average of 4-10 orioles per 40 km (25 mi) route (Sauer et al. 2003). Brown-headed (Molothrus ater) and Bronzed (M. aeneus) cowbirds have been found parasitising Bullock’s Oriole, but their impact on the species is reduced by the oriole’s habit of ejecting cowbird eggs from their nests.

Since Bullock’s Oriole is a common and widespread breeder in Texas and since population reductions found by the BBS for this species are relatively small (Sauer et al. 2003), Bullock’s Oriole will probably remain a member of the Texas avifauna for the foreseeable future. Text by Robert C. Tweit (2005).

Literature cited:
American Ornithologists’ Union. 1983. Checklist of North American birds, 6^thed. Am. Ornithol. Union, Washington, DC.

Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.

Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.

Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.

Rising, J. D., and P. L. Williams. 1995. Bullock’s Oriole (Icterus bullockii). In The birds of North America, No. 416 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.

Sauer, J. R., J. E. Hines, and J. Fallon. 2003. The North American Breeding Bird Survey, results and analysis 1966-2002. Version 2003.1. USGS Patuxent Wildlife Research Center, Laurel MD (Web site, http://www.mbr-pwrc.usgs.gov/bbs).