The Giant Cowbird is one of the larger species of icterid. It is wholly black with long and pointed wings, a rather long and rounded tail, a stout black bill with broad and flattened frontal shield, and with the feathers of sides of neck developed into erectile lateral ruffs. The bill is about as long as head, and is stout and subconical. The wing is about > 5x as long as culmen, ca. 4.5x as long as tarsus. Primary 9 (P9) longest or same as P8, P7 much shorter. Inner webs of outer primaries gradually narrowed terminally. Tail about three-fourths as long as wing, rounded, rectrices with broad ends. Tarsus with anterior scutella distinct. Feathers of neck greatly developed laterally and posteriorly, forming a conspicuous ruff in males, less so in females, with bare or downy space immediately behind on sides of neck.
The male Giant Cowbird may be confused at a distance with the Great-tailed Grackle (Quiscalus mexicanus), which is of similar size, but the cowbird has a stouter build overall, with a "thick-necked" and small-headed appearance due to the conspicuous ruff of feathers around the neck. The cowbird also has a shorter and stouter bill, and a shorter tail lacking the characteristic grackle "keel." Unlike the steady flapping flight of the grackle, the cowbird's flight is punctuated by a short pause after every few wingbeats, resulting in a short, slightly descending glide (Skutch 1954, Hilty and Brown 1986, Ridgely and Greenfield 1989, Skutch and Stiles 1989, Howell and Webb 1995, Ridgely and Greenfield 2001, Hilty 2003).
The female Giant Cowbird is smaller than the Great-tailed Grackle, but also is significantly larger than otherwise similar blackbirds, such as Shiny Cowbird (Molothrus bonariensis) and Melodious Blackbird (Dives dives).
The following information based on Ridgway (1902) and on Jaramillo and Burke (1999).
Adult, male: General color black. Feathers of head, neck and body more or less broadly tipped uniformly with glossy violet, and back tending to bronzy; on underparts forming a sharply defined terminal band or margin to each feather -- basal black conspicuously exposed and violet tips becoming narrower posteriorly and less distinct on rump than on under parts. The smaller wing coverts, tail coverts and thighs uniform black or with indistinct bluish violet margins. Larger wing coverts, remiges and rectrices faintly glossed with bluish. Alternate and basic plumages similar.
Adult, female: Much duller than male. Black, with tips of feathers of head, neck and body indistinctly tipped with more glossy violaceous-black.
Immature: Similar to the adult of the corresponding sex, but duller and less iridescent; iridescence completely absent on the belly, vent, and thighs, and elsewhere may be patchy (since only the tips of feathers are glossy). Immature males are smaller than adults, and their primaries are shorter and less pointed. In both sexes, immatures show greater wear on the primaries.
Juvenile: Blackish, but the plumage has no iridescence at all. Also the irides are dark (brown or grayish) and the bill is pale (horn colored), darkening with age (see Bare Parts).
Hatchlings: Young altricial (nearly naked and helpless) and nidicolous (confined to nest). Recently hatched ("2 or 3 days old") cowbird: skin white; occipital, humeral, dorsal and lateral tracts, and slight scattering on thighs, well covered with long, dark gray down; beak white and gape wide and yellowish (Crandall 1914). Another young at "about two weeks old"; feathers black, legs and feet black, iris dark hazel; beak and bare portions of face, including lore, entirely white; gape pale yellow (Crandall 1914).
Bill and gape: black (Ridgway 1902).
Iris. Described as variable, yellow in west and south parts of distribution, orange or orange-red to northeast (Guianas, etc.; Ridgely and Tudor 1989). Yellow or yellowish white in S. o. violea; red in S. o. mexicana (Ridgway 1902). Red in males, yellowish red in females (ffrench 1980). "Reddish or reddish-brown" (Phillipps 1899). Bright red (with post mortem changes within seconds to orange and then to pale straw-yellow; Olson 1973).
Legs and feet: both black (Ridgway 1902).
Most data are from the northern portions of the range of the species.
Specimens of impacificus from Mexico (and Belize?; Ridgway 1902):
Wing length, male: mean 189 mm (range 177-202.7 mm, n = 6)
Wing length, female: 160 mm (n = 1)
Tail length, male: mean 145.8 mm (range 133.4-151.6 mm, n = 6)
Tail length, female: 118.9 mm
Culmen (from base), male: mean 37.1 mm (range 35.8-37.6 mm, n = 6)
Tarsus length, male: mean 42.9 mm (range 40.6-43.9 mm, n = 6)
Tarsus length, female: 38.9 mm (n = 1)
Specimens of nominate oryzivorus from Colombia and Panama; Ridgway 1902):
Wing length, male: mean 201.2 mm (range 179.3-208.3 mm, n = 12)
Wing length, female: mean 157.5 mm (range 154.2-161.3 mm, n = 4)
Tail length, male: mean 152.4 mm (range 135.4-159.3 mm, n = 12)
Tail length, female: mean 117.3 mm (range 112.5-122.4 mm, n = 4)
Culmen (from base), male: mean 37.8 mm (range 36.6-39.9 mm, n = 12)
Culmen (from base), female: mean ? mm (range 31.2-34 mm, n = 4); the mean was reported as 40.9, which surely is a mistake
Tarsus length, male: mean 44.2 mm (range 45-48 mm, n = 12)
Tarsus length, female: mean 40.4 mm (range 39.9-40.9 mm, n = 4)
Specimens of nominate oryzivorus from Colombia and Panama; Wetmore et al. 1984):
Wing length, male: mean 191.1 mm (range 169.9-204.5 mm, n = 10)
Wing length, female: mean 155.2 mm (range 145.5-167.0 mm, n = 10)
Tail length, male: mean 145.4 mm (range 129.5-157.0 mm, n = 10)
Tail length, female: mean 119.2 mm (range 112.5-133.0 mm, n = 10)
Culmen (from base), male: mean 37.5 mm (range 35.9-39.9 mm, n = 10)
Culmen (from base), female: mean 32.7 mm (range 31.3-33.5 mm, n = 10)
Tarsus length, male: mean 44.5 mm (range 41.7-48 mm, n = 10)
Tarsus length, female: mean 38.8 mm (range 37.2-39.9 mm, n = 10)
No population samples; overall range for males 174-242 g, as follows:
One (Belize), 211 (Russell 1964); four (Panama), 227, 228, 233, 242 (Burton 1975); one (Trinidad), 175 (ffrench 1980); one (Suriname), 175 (Haverschmidt 1966); one (French Guiana), 174 g (Dick et al. 1984); two (Argentina), 205, 213 (Contreras 1983).
Even fewer data for females:
One (Trinidad), 144 (ffrench 1980); three (Suriname), mean 129, range 120 - 140 (Haverschmidt 1966).
The measurements given in Smith (1979) – e.g., males about 120 g, females about 74 g – seem greatly out of line in comparison to other sources.