Screaming Cowbird Molothrus rufoaxillaris

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




Walking, hopping, climbing, etc. No information.

Flight. No information.

Swimming and diving. Not known to swim.


Preening, head-scratching, stretching, bathing, anting, etc. Usually forages on ground. Preening movements not specifically described but likely similar to those used by most other passerines. No reports of anting.

Sleeping, roosting, sunbathing. No information.

Daily time budget. No known quantitative studies.


Physical interactions. No information.

Communicative interactions. No information. Other Molothrus cowbirds (Brown-headed (M. ater), Bronzed (M. aeneus), Shiny (M. bonariensis) and Giant (M. oryzivorus) cowbirds) have a Head-Down, or Preening Invitation, display as does the Bay-winged Cowbird (Agelaioides badius); the Preening Invitation display of the Bay-winged Cowbird probably has a similar function but different posture than that seen in Molothrus cowbirds (Selander 1964).


Territoriality. Does not seem territorial; maintains overlapping home ranges (Fraga 1986).
Individual distance. No information.

Sexual Behavior

Mating system and sex ratio. Sex ratio unity based on trapping data (27 females : 33 males; Mason 1987).

Pair bond. Apparently monogamous and remaining paired through breeding season (Friedmann 1929, Mason 1987); 3 pairs together for 4 months, 1 pair left study area after 3 month, 2 pairs broke up due to death of mate, 1 female remated (Mason 1987). Banded pair always seen together between 11 November 1977 and 13 March 1978 (Fraga 1998). Males remain close to females (Mason 1987) and guard their mates (Fraga 1986).

Courtship displays. Song Spread (Fraga 1986) display likely analogous to the "Bow," "Topple-Over," or "Song Spread" of Brown-headed Cowbird (Molothrus ater; see Lowther 1993) and other icterids, identified by Friedmann (1929) as the "commoner display". Displays from mid-September to mid-November; apparently courtship ends about 1 month before breeding (Friedmann 1929). Male fluffs feathers of head and body, brings tail forward and under body, arches wings to a nearly horizontal position and quivers; after quivering about 1 s (with no forward bowing) emits a "very rusty but quite explosive squeak dzeee, quickly followed by pe tzee (2 syllables, first very short, second long and drawn out). The quivering and mode of delivery gives "impression that the dzeee note was stuck in his throat and was finally spit out". "Other display" (Friedmann 1929) is likely a variation on the first, differing in intensity and in being oriented towards female: similar to "commoner" except that while giving dzeee, bird hops up and down on its perch, sometimes moving closer to female with each jump. This display given perched in tree, not seen on ground (Jaramillo and Burke 1999).

Solicitation (pre-copulatory) display of female not described but similar to that of other icterids; for 8 copulations observed, male song spread preceded female solicitation; copulations occurred in morning (between 07:23 - 10:47) within 25 m of Bay-winged Cowbird nest (not always active, Fraga 1998).

Extra-pair copulations. No information. Male-male fights were seen twice by Fraga (1986) when second male approached female closer than mated male; one fight occurred after female made pre-copulatory display; copulation occurred after intruding male left (Fraga 1986).

Social and interspecific behavior

Degree of sociality. Almost always seen in pairs or small flocks (Hoy and Ottow 1964), pairs inseparable during breeding season, even when in flocks (Jaramillo and Burke 1999). Nest visiting is sometimes a gregarious activity, 5 pairs have been observed visiting a nest at the same time (Mason 1987); host nests may receive several parasitic eggs in a single day (Mason 1987).

Play. No information.

Nonpredatory interspecific Interactions. Male was once seen to Bill-tilt to male Shiny Cowbird (Molothrus bonariensis) while feeding (Friedmann 1929). Will be chased by Bay-winged Cowbird (Agelaioides badius) males and helpers from their nest sites (Fraga 1979, 1991, 1992). Bay-winged Cowbirds are cooperative breeders (Fraga 1972, 1991), with up to 18 adults attending fledglings and young Screaming Cowbirds, if part of a Bay-winged Cowbird brood, receive similar attention; morphological similarity of Screaming Cowbirds to Bay-winged cowbirds presumably provide an advantage in remaining with social Bay-winged Cowbirds (Fraga 1979).

Non-breeding Screaming Cowbirds usually in mixed flocks of other icterids (including 4 other species; only Bay-winged Cowbird identified; Fraga 1998). Shared roost sites with Bay-winged Cowbirds; counts of 30 mixed flocks give total 396 Bay-winged : 85 Screaming (3.7 : 1 ratio which also reflects relative abundance, Fraga 1998).
Visits to nest sites may be part of pair formation in this species; domed nests and cavities that could be used by Bay-winged Cowbirds visited, even if active and in use by other species (e.g., Rufous Hornero [Furnarius rufus] and Great Kiskadee [Pitangus sulphuratus]) which usually attack the visiting Screaming Cowbird (Fraga 1998).


Kinds of predators. Predators of adults likely to be similar as those of other small, woodland birds.
Response to predators. During 12 d of observation, Aplomado Falcons (Falco femoralis) made 34 attacks on mixed flocks of Bay-winged (Agelaioides badius) and Screaming cowbirds. Bay-winged Cowbirds gave hawk-alarm calls that immobilized the whole flock, while Screaming Cowbirds remained silent; both species also were observed to respond similarly to "other predators" (Fraga 1986, 1998).

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.