Screaming Cowbird Molothrus rufoaxillaris

  • Order: Passeriformes
  • Family: Icteridae
  • Monotypic
  • Authors: Peter E. Lowther


Distinguishing Characteristics

Small, brood parasitic icterid; mass 50-60 g. Sexes similar but noticeably dimorphic in size. Black with only slight greenish blue sheen; axillars rufous, but these usually are not apparent in the field (Ridgely and Tudor 1989). The eggs, nestlings, and juveniles are very similar to the corresponding stages of the Bay-winged Cowbird (Agelaioides badius); see Similar Species.

Similar Species

Adults are similar to the male Shiny Cowbird (Molothrus bonariensis), which has a strong violet gloss; Screaming Cowbirds only infrequently flock with Shiny Cowbirds (which are sexually dimorphic in plumage), thus the lack of sexual dimorphism in pairs or groups indicates Screaming Cowbirds (Ridgely and Tudor 1989). Furthermore, the Shining Cowbird has a longer bill, and a more rounded shape to the head (Jaramillo and Burke 1999).

Screaming Cowbirds are very similar to their principal host species, the Bay-winged Cowbird, at several stages of their life cycle (eggs, nestlings, and juveniles), but differences still evident (Fraga 1979). At hatching, the reddish skin of the Bay-winged Cowbird soon dries to orange, and the bill is pinkish with a dark area around the white eggtooth. The skin of the hatchling Screaming Cowbird becomes pink or pale pink, and the bill is pinkish without a dark area around the eggtooth. Bills of postfledging Screaming darken to black, while Bay-winged Cowbird young retain dark brownish bills with lighter tips which slowly darken to black in 2-3 months. A size difference also may be evident, as Bay-winged Cowbird adults average 44.48 g ± 0.65 SD (range 39 - 51, n = 24; Fraga 1979) and some Screaming Cowbird fledglings can exceed this range. Bay-winged Cowbirds may solicit preening by assuming "Allopreening Invitation Posture" [= Head-Down display (?); see Selander 1964] which Screaming Cowbirds have not been seen to do (Fraga 1979); young Screaming Cowbirds may utter a version of their unmistakable adult vocalization (Fraga 1979).

Detailed Description

Adult (Definitive Basic): Sexes alike. Entire body, head, wings and tail dark black with dull (not shiny) gloss tinged with deep purplish blue-black, wings and tail faintly glossed with greenish, some axillars bright chestnut (Friedmann 1929).

Immature (Formative plumage). Similar to adult, but retains some juvenile underiwng wingcoverts, and sometimes wing coverts and scapulars (Jaramillo and Burke 1999).

Juvenile (First Basic plumage). Head, neck, mantle, scapulars, interscapulars, back, rump grayish-buffy-brown; upper tail coverts olive-brown; tail feathers very dark olive-brown; lesser wing coverts sandy buffy-brown; middle coverts light grayish rufous; greater coverts dark fuscous broadly edged with rufous; primaries and secondaries dark fuscous externally edged with light rufous chestnut; under wing coverts pale isabelline gray; sides of head grayish buffy-brown; lores blackish brown; entire underparts sandy olive buffy-brown.

Hatchlings. Natal down mouse-gray (Friedmann 1929).


Definitive Basic plumage is acquired by a complete Prebasic molt, March-April (Jaramillo and Burke 1999).

Formative plumage is acquired by an incomplete Preformative molt, December-February, involving most of the plumage except some underwing coverts, and some wing coverts and scapulars (Jaramillo and Burke 1999).

Juvenile plumage is retained for only a few weeks, with the Preformative molt commencing at age 34-44 days (Fraga 1996, Jaramillo and Burke 1999).

Bare Parts

Iris: Brown to grayish brown (Belton 1985).

Bill and gape: Black (Friedmann 1929); bill black (Belton 1985).

Tarsi and toes: Black (Friedmann 1929, Belton 1985).


Males (n=3): wing, mean 117 mm (range 116-118); tail, mean 86 mm (range 85-87); culmen, mean 15.7 mm (range 15-16)

Females (n=4): wing, mean 105.5 mm (range 104-107); tail, mean 76.5 mm (range 74-78); culmen, mean 15.0 mm (14.5-15.5; Navas 1970).

Hippocampus volume (estimated from figure): 29 mm3 (n=6) for females, 27 mm3 (n=6) for males.

Telencephalon volume: 850 mm3 for both sexes (Reboreda et al. 1996).

Mass of 4 males: 66, 64, 62, 61 g; mass of 5 females: 52, 51, 51, 49, 48 g (Fraga 1979). Males 58.0 g ± 4.0 SD (range 46.5 - 66; n=33), females, 48.4 g ± 4.4 SD (range 42-64; n=27; Mason 1980, 1987). Males, mean 55.5 g ± 1.5 SD (n=6), females, mean 52.3 g ± 1.0 SD (n=6; Reboreda et al. 1996). Males, mean 56.6 g (range 55.1-57.5; n=3); females, mean 42.6 g (range 36.1-48.5; n=4; Navas 1970). Males, 56-65 g (n=4); females, 38 g, 57 g (Belton 1985).

Recommended Citation

Lowther, P. E. (2011). Screaming Cowbird (Molothrus rufoaxillaris), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.