Icterus bullockii
Bullock's oriole

Bullock’s orioles (Icterus bullockii) are native to western North America, especially most of the western United States. They are common throughout most of their range (Jaramillo and Burke 1999). They are found as far north as southern British Columbia during the breeding season, (Jaramillo and Burke 1999; Rising and Williams 1999) but the winter range is largely confined to Mexico (Jaramillo and Burke 1999; Howell and Webb 1995). Though no geographic boundary hinders movement to the east, the range generally ends in the western counties of North Dakota, South Dakota, Kansas, and in central Texas (Jaramillo and Burke 1999; Rising and Williams 1999). The breeding range extends into southern Canada, including Vancouver Island, British Columbia, Alberta, and Saskatchewan (Jaramillo and Burke 1999; Rising and Williams 1999). Bullock’s orioles also breed in Durango, Sonora, and Baja states of northern Mexico (Howell and Webb 1995; Jaramillo and Burke 1999). Bullock’s orioles are absent from southwestern Arizona and high elevation, arid portions of Idaho (Rising and Williams 1999). During winter, Bullock’s orioles may be found throughout Mexico, from Sinaloa south and east through central Mexico to Oaxaca. They are occasionally seen during the winter months in Baja California and in Guatemala (Howell and Webb 1995; Jaramillo and Burke 1999; Rising and Williams 1999). (Howell and Webb, 1995; Jaramillo and Burke, 1999; Rising and Williams, 1999)

Bullock’s orioles prefer riparian corridors, open woodland, and scrub forest (Jaramillo and Burke 1999). During the breeding season, they are observed in patchy forest dominated by cottonwood and pecan throughout much of their range (Jaramillo and Burke 1999; Rising and Williams 1999). Willows (Salix) are preferred in riparian areas; by contrast, in arid southwestern Arizona this species prefers scrub forest trees, especially mesquite (Prosopis) and salt cedar (Tamarix) (Rising and Williams 1999). During the winter, Bullock’s orioles are often seen foraging in urban areas, particularly parks. In California, eucalyptus (Eucalyptus) is used as a source of nectar (Jaramillo and Burke 1999; Rising and Williams 1999). (Jaramillo and Burke, 1999; Rising and Williams, 1999)

Bullock’s orioles are medium-sized orange orioles. This species is sexually dimorphic, with males being slightly larger than females. Males range in length from 17.2 to 19.3 centimeters, while females are from 16.8 to 19.1 centimeters. Bullock’s orioles range in mass from 29 to 43 grams (Rising and Williams 1999). The culmen is straight and mostly black; the lower edge of the culmen is blue (Jaramillo and Burke 1999; Sibley 2000). In adults of this species, the lower mandible is bluish gray. All exposed skin is bluish gray (Jaramillo and Burke 1999). In adults, the tail is square and slightly graduated (Sibley 2000). (Jaramillo and Burke, 1999; Rising and Williams, 1999; Sibley, 2000)

During the breeding season, adult males are characterized by their strongly contrasting black and orange plumage and black throat patch, though older females may also display a small throat patch. Though the crown is black, the rest of the head is orange, with a black eye-line that extends to the crown (Jaramillo and Burke 1999; Rising and Williams 1999; Sibley 2000). The supercilium is bright orange. The underparts and breast range in color from yellow to orange while the back is black (Jaramillo and Burke 1999; Pyle and Howell 1997; Rising and Williams 1999; Sibley 2000). Although the tail is mostly black, the outer three or four retrices are tipped orange, forming a dark T shape beginning at the base of the tail (Jaramillo and Burke 1999; Sibley 2000). The wing coverts are edged in white, forming a wing panel. The scapulars are black (Jaramillo and Burke 1999; Rising and Williams 1999). (Jaramillo and Burke, 1999; Rising and Williams, 1999; Sibley, 2000)

Adult females are duller and, in most cases, lack the dark throat patch seen in male Bullock’s orioles (Jaramillo and Burke 1999; Pyle and Howell 1997). The upperparts, face, and breast are a dull grayish yellow with indistinct dark streaks on the back. The underparts are grayish, ranging in color from white to yellow (Jaramillo and Burke 1999; Rising and Williams 1999; Sibley 2000). The coverts are fringed white, and do not form a solid wing-panel as in the male, while the secondaries and tertials are fringed gray. The crown is olive. Alternatively, adult females may have a small black throat patch, though they lack the black lores seen in adult males. It is hypothesized that females with throat patches are older individuals (Jaramillo and Burke 1999). (Jaramillo and Burke, 1999; Pyle and Howell, 1997; Sibley, 2000)

Immature males resemble adult females, though in adult females the wings show less abrasion. Immature females are similar to juveniles of both sexes. Immature females can be distinguished from juveniles by the presence of worn, dark-tipped outer coverts, fresh white-tipped inner coverts, and brown medial coverts that are tipped white. Juveniles cannot be sexed in the field through differences in plumage. In general, juveniles resemble adult females, but with darker wings, less-extensive wing bars, the lack of a malar (throat) patch, and the presence of a pink mandible (Jaramillo and Burke 1999). (Jaramillo and Burke, 1999)

Bullock’s orioles are seasonally monogamous, forming pair bonds that last for a single breeding season (Rising and Williams 1999). (Rising and Williams, 1999)

Males arrive at the breeding ground approximately two weeks before females. Breeding begins in May and continues until mid July. The breeding season varies geographically and tends to begin and end later in the west and in northern latitudes (Jaramillo and Burke 1999; Harrison 1978; Rising and Williams 1999). (Harrison, 1978; Jaramillo and Burke, 1999; Rising and Williams, 1999)

Bullock’s oriole nests are deep, pensile structures and are usually suspended on the outer branches of a tree (Harrison 1978). Nests are on average 6 inches (15 cm) deep and 4 inches (10 cm) in diameter, the walls of which are approximately 0.75 inches (1.9 cm) thick (Bent 1958). The nest opening is contracted and oval-shaped (Bent 1958). The nest is constructed primarily of plant fiber, especially flax fiber, oat stalks, and the interior bark of willow and juniper trees; horsehair and twine are also used when the nest is built near human developments. The nest is lined with plant down, wool, hair, and moss (Bent 1958; Harrison 1978). Both male and female have been observed to participate in nest construction, though normally only the female weaves the nest (Bent 1958). Nests are abandoned at the end of the breeding season and are not reused (Sibley 2000). (Bent, 1958; Harrison, 1978; Sibley, 2000)

Eggs are laid in clutches of 3 to 6, with clutches of 4 or 5 eggs being the most common (Harrison 1978; Sibley 2000). The eggs themselves are small, approximately 2.5 cm long by 1.5 cm in diameter. Nest size is variable, and according to Schaefer (1976), nests range in depth from 10 cm to 38 cm. Eggs are whitish with dark purple or brown scrawling patterns near the wide end of the egg (Harrison 1978). (Harrison, 1978; Sibley, 2000)

Only the female incubates the eggs, which hatch after approximately two weeks (Harrison 1978). Nestlings are altricial: they are entirely reliant on parental care for their survival (Sibley 2000). Both sexes care for the young, providing them with food as well as defending the nest from predators (Bent 1958). (Bent, 1958; Harrison, 1978; Sibley, 2000)

The current observed maximum lifespan in the wild is 7 years, most individuals do not survive this long (USGS: Patuxent Wildlife Research Center). (Klimkiewicz and Futcher, 1987)

Bullock’s orioles are most often seen in pairs during the breeding season, occasionally forming small flocks. They are territorial during the breeding season, males aggressively defending nesting sites (Rising and Williams 1999). Although the nest is defended from conspecifics, mutual feeding grounds are usually unguarded. Although territoriality has not been extensively studied during the winter months, Bullock’s orioles will defend flowering Erythrina breviflora, a source of nectar, from other species, particularly hummingbirds (Trochilidae) (Jaramillo and Burke 1999; Rising and Williams 1999). While breeding pairs nest singly, nests tend to be clustered. Breeding density tends to be highest in areas where there food is most abundant. Bullock’s orioles migrate at night, forming small flocks of up to ten individuals (Rising and Williams 1999). (Jaramillo and Burke, 1999; Rising and Williams, 1999)

A family of Icterus bullockii living in Kansas was observed to have a home range of 0.85 hectares (Fitch 1958; Zeiner et al. 1988). (Fitch, 1958; Zeiner and Laudenslayer Jr., 2009)

Vocalization forms the primary method of communication for Bullock’s orioles. Vocal communications include two short, energetic songs described by Dawson (1923) as kip, kit-tick, kit-tick, whew, wheet and cut cut cudut whee up chooup; the latter sequence is less common than the former. In the latter sequence, the last note is often unclear (Bent 1958). Both males and females sing, female song being lower in pitch and less sweet than that of the male. Females are most likely to sing earlier in the season, and have been observed to sing while constructing the nest. Males sometimes give a short cry while with their mate (Bent 1958). Baltimore and Bullock’s orioles have similar songs, though Baltimore orioles tend to have harsher songs than Bullock’s orioles (Bent 1958; Wheelock 1903). (Bent, 1958; Dawson, 1923; Wheelock, 1903)

Bullock’s orioles are primarily insectivorous. During the breeding season, they feed mostly on butterflies and their larvae (Lepidoptera) and augment their diet with beetles (Coleoptera), weevils (Curculionoidea), and scale insects (Coccoidea). Bullock’s orioles occasionally eat mollusks and small lizards. In one case, a member of this species was observed killing and eating a hummingbird. Fruit is also a major food source, accounting for approximately 40% of this species diet during the summer months (Sibley 2001). Stomach content analyses indicate that Bullock’s orioles especially favor cherries, blackberries, raspberries, and figs (Bent 1958). Bullock’s orioles either glean insects from leaves or forage in clearings. One behavior, common to the icterids, is called gaping: the bill is inserted into soil or a plant stem and then forced open. This behavior leaves an opening that may uncover prey species (Sibley 2001). (Bent, 1958; Sibley, 2001)

Predators include jays, magpies, crows, and squirrels (Bent 1958; Jaramillo and Burke 1999). All four feed on the eggs or young of Bullock’s orioles. Nests are sometimes parasitized by cowbirds; however, Bullock’s orioles seem to be better able to identify parasitic eggs. They remove cowbird eggs by puncturing the shell with the bill and removing the egg from the nest, though an individual removing cowbird eggs in this manner risks damaging its own eggs (Bent 1958). (Bent, 1958; Jaramillo and Burke, 1999)

Bullock’s orioles help to spread the seeds of several plant species. Their eggs and young are a food source for several species and help keep insect populations in check. Please see the previous sections for more detailed information, especially “Food Habits” and “Predation”.

Bullock’s orioles regulate the populations of black olive scale insects (Parlatoria oleae), which in large numbers have the potential to destroy olive crops. Bullock’s orioles also consume cotton boll weevils (Anthonomus grandis), insects that can negatively impact cotton production (Bent 1958; Howell 1907). Bullock’s orioles are not known to have any significant negative impact on crop production. ("The relation of birds to the cotton boll weevil", 1907; Bent, 1958)

Bullock's orioles do not have any significant negative economic impact.

Bullock’s orioles have declined in population size in North America since the 1960s, though this species is not considered endangered (Sibley 2001). Human factors contribute to this decline, through habitat degradation and the use of insecticides. An increase in habitat availability, caused by the planting of shelterbelts and by human control of brush fires, has probably led to population stability in the Great Plains. However, irrigation has reduced available water supplies in some parts of Kansas and Colorado, which likely negatively affects the carrying capacity for Bullock’s orioles in these regions. Unlike most migratory, riparian species, Bullock’s orioles have not seen a population decline in the Colorado River Valley (Rising and Williams 1999) (Rising and Williams, 1999; Sibley, 2001)