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Screaming Cowbird Molothrus rufoaxillaris

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



The Bay-winged Cowbird (Agelaioides badius) is the primary host of the Screaming Cowbird. Few convincing reports exist of other species serving as hosts.

Screaming Cowbird distribution closely matches the distribution of Bay-winged Cowbirds (subspecies A. b. badius and A. b. bolivianus only; the subspecies A. b. fringillarius of northeastern Brazil is allopatric). Basic information on biology of the Bay-winged Cowbirds is found in the works of Fraga (1972, 1979, 1983, 1984, 1986, 1988, 1991, 1992, 1998; see also De Mársico and Reboreda 2008, Lowther 2010). Most nesting attempts of Bay-winged Cowbird are parasitized by the Screaming Cowbird: 75 nests parasitized out of 85 nests at one Argentina site and 16 of 17 nests at another (Fraga 1986); 13 of 14 nests (Hoy and Ottow 1964), 15 of 15 nests (Mason 1980), and all of 52 nests (Jaramillo 1993). Most nests are also multiply parasitized by Screaming Cowbirds with "usually" 2 Screaming Cowbird eggs (Friedmann 1929: 49), 1 to 12 (Fraga 1986), 15 (Hoy and Ottow 1964) or 19 eggs (Mason 1980). Screaming Cowbirds may often lay eggs in Bay-winged Cowbird nests before host eggs are laid; these are ejected by Bay-winged Cowbirds (Hoy and Ottow 1964, Mason 1980, Fraga 1986). Mason (1980) estimated that 87% of Screaming Cowbird eggs were laid before Bay-winged egg laying and would therefore be ejected or deserted. At 49 nests with known egg laying information, 28 of 189 Screaming Cowbird eggs were laid from 1-20 d before the host's clutch and eventually were ejected (Fraga 1986). Parasitic eggs laid during the host's laying period usually are accepted unless the total clutch size becomes too large (Hoy and Ottow 1964, Mason 1980, Fraga 1986). Screaming Cowbirds also remove or puncture eggs that may be in the nest (host and parasite) when parasitizing a nest; 77 of 197 Bay-winged Cowbird eggs in parasitized nests were punctured (Fraga 1986). Of 101 nests in which Bay-winged Cowbirds began laying, 94 were parasitized (with 441 eggs) as well as 11 of 13 nests (with 52 eggs) which were deserted by Bay-winged Cowbirds before egg laying began (De Mársico and Reboreda 2008).

Chopi Blackbird (Gnorimopsar chopi).– First convincingly shown as a Screaming Cowbird host by Sick (1985, 1993); 5 cowbird chicks were found in 3 blackbird nests in southeastern Brazil, near Rolandia, Paraná, in an area recently colonized by Screaming Cowbirds and where Bay-winged Cowbirds were not present. Details of contents of these 3 nests were 1 blackbird plus 3 Screaming Cowbirds; 0 blackbird plus 1 Screaming Cowbird plus 3 Shiny Cowbirds (Molothrus bonariensis); 1 blackbird plus 1 Screaming plus 3 Shiny Cowbird. In Misiones Province, Argentina, Fraga (1996) also found Chopi Blackbird nests parasitized: 1 nest found with 2 blackbird and 1 cowbird young; also 2 of 4 family groups seen included cowbird young (0 blackbird plus 1 cowbird; 1 blackbird plus 2 cowbirds). Parasitism of Chopi Blackbirds had been indicated by Azara (1802; see also Laubmann 1939, Pereyra 1945). It is possible that Screaming Cowbirds are more likely to use this species as host when not sympatric with Bay-winged Cowbirds (Fraga 1996; supposition by Robbins et al. 1999). Chopi Blackbirds have been noted to attack and chase cowbirds that visited their nests (Fraga 1996).

Brown-and-Yellow Marshbird (Pseudoleistes virescens).– In Buenos Aires Province, Argentina, for the 1993 - 1994 breeding season: 17 of 338 nests of the Brown-and-Yellow Marshbird were parasitized by Screaming Cowbirds: 15 nests contained 1 cowbird egg; 2 with 2 cowbird eggs (of these 17 nests, 14 were parasitized also by Shiny Cowbird: 4 nests with 1 Shiny Cowbird egg, 10 with 2-5 Shiny Cowbird eggs); of the 17 parasitized nests, 14 were predated; 1 Screaming Cowbird fledged from a nest at 37 g at 8 d of age (Mermoz and Reboreda (1996); for 4 breeding seasons (1994 - 1997 data), of 382 marshbird nests, 236 parasitized (193 parasitized by Shiny Cowbird only, 12 by Screaming Cowbird only, and 31 by both cowbird species; marshbird nests received 54 Screaming Cowbird eggs and 437 Shiny Cowbird eggs; Mermoz and Fernández 2003). Grant (1911) had listed this species as a host, but his report was considered a misidentification of Shiny Cowbird parasitism by Friedmann (1929).

Several species have been  described erroneously as hosts of the Screaming Cowbird. Friedmann (1963) thought these errors were of misidentifications of Shiny Cowbird eggs. Both Mason (1980) and Fraga (1986) give full support for Friedmann's opinion. These supposed hosts (and initial references) are Chalk-browed Mockingbird (Mimus saturninus) and Yellow-winged Blackbird (Agelasticus thilius; Grant 1912); Rufous Hornero (Furnarius rufus), Great Antshrike (Taraba major), Great Kiskadee (Pitangus sulphuratus), Rufous-bellied Thrush (Turdus rufiventris), and Saffron Finch (Sicalis flaveola [pelzelni]; Pereyra 1938). Those egg sets reported by Pereyra were examined by Fraga (1986) who found only 1 of the 12 records to be apparently correct. This possible set is of a Rufous Hornero (Furnarius rufus) taken 28 Nov 1931 at Manchala, Tucuman, Argentina, by P. Girard and consists of 2 host eggs and 1 “Screaming Cowbird” egg.

Details of host selection are little known but there is some suggestion that the continued use of "new" host species may be due to females specializing in selecting these new hosts.  Analysis of mitochondrial DNA of 58 eggs or young (from 27 Bay-winged Cowbird and 31 Chopi Blackbird nests) showed that among 11 identified haplotypes -- 3 of which rare and identified only in adult Screaming Cowbirds -- 3 were unique to young taken from Bay-winged Cowbird nests and 2 unique to young taken from Chopi Blackbird nests (Mahler et al. 2009).


Pair formation. No information. One female apparently paired when 133 d old (Fraga 1986).

Nest building. Does not build nest.

First/only brood per season. In Buenos Aires Province, Argentina, Bay-winged Cowbirds nest from November to early March (Fraga 1979). Egg dates: 18 November-15 March (Fraga 1998); 8 October-18 December in nests of Brown-and-yellow Marshbirds (with egg dates 29 September-16 December; Mermoz and Fernández 2003).


Brood parasite, does not build own nest. Parasitic primarily on Bay-winged Cowbird which is sometimes a "nest parasite" – i.e., it takes over nests of other species – though it incubates its own eggs and cares for its own young. Bay-winged Cowbirds usually builds their own nests or make use of old nests of other species (e.g., nests of Anumbius, Pseudoseisura or Myopsitta), rarely pirates nests (e.g., Little Thornbird Phacellodomus sibilatrix) and have used nest boxes (Fraga 1988).
This species observed to visit nests of other species, but these nests are sites appropriate for Bay-winged Cowbirds (see Fraga 1998: Table 9.2). In nest searching, banded pair observed at 12 different nests of Bay-winged Cowbirds between November and March; 2 nests 860 m apart were visited in same day (Fraga 1998). Breeding season nest visits average 2.33 visits/h (range 0-9, n=63 watches as 21 nests; Fraga 1998). Nest visits noisy and conspicuous: 1 or more pairs land ≥10 m from nest with audible wing flaps, male song spreads or calls while flicking tail; may be attacked by Bay-winged Cowbird if approach nest (Fraga 1998).


Shape. Usually ovate, more rounded than eggs of Bay-winged Cowbird.

Size. Mean egg size: 23.184 mm ± 0.966 SD (range 20.05-25.80; n=255) x 17.842 mm ± 0.644 SD (range 15.55-19.35; n=225; Fraga 1983). (For comparison, mean size of Bay-winged Cowbird eggs: 23.678 mm ± 0.731 SD, range 21.35-25.30; n=226 x 17.657 mm ± 0.582 SD, range 16.50-19.10; n=226; Fraga 1983). In separating Bay-winged and Screaming cowbird eggs, Screaming Cowbird eggs are more spherical; background color and presence/absence of thick dark scrawls were most useful criteria in identifying eggs (Fraga 1983). Mean size: 22.4 mm (range 20.7 - 24.1) x 17.6 mm (range 16.7-18.6); egg shell mass 0.310 g (range 0.270-0.350, n=61; Hoy and Ottow 1964). For comparison, mean size of Bay-winged Cowbird eggs: 24.0 mm (range 21.0-25.4) x 18.1 mm (range 16.5-19.1); egg shell mass, 0.270 g (range 0.230-0.305, n=43; Hoy and Ottow 1964). Relative shell thickness, q = LB/m, where L = length, B = breadth, m = shell mass in mg (see Rey 1892), is 1.28 (range 1.15-1.50) (and for comparison, mean relative shell thickness of Bay-winged Cowbird eggs: 1.62, range 1.49-1.79; Hoy and Ottow 1964). Mean size: 23.2 x 17.6 mm (n=10; Mason 1985; for comparison, mean size of Bay-winged Cowbird eggs: 23.6 x 18.6 mm, n=36).

Mass. Mean mass 4.04 g ± 0.38 SD (range 3.00-5.20, n=117; Fraga 1983). (For comparison, mean mass of Bay-winged Cowbird eggs, 4.03 g ± 0.33 SD (range 3.20-4.80; n=113; Fraga 1983). Mean 4.2 g (n=10; Mason 1985; for comparison, mean mass of Bay-winged Cowbird eggs: 4.5 g, n=10).

Color. Differences in coloration and markings can safely separate host Bay-winged and Screaming cowbird eggs (Fraga 1983). Results of Fraga (1983) agree with Hartert and Venturi (1909) but neither with Hudson (1920) nor with Friedmann (1929) regarding egg coloration. See also Smyth 1928.

Ground color mainly white to creamy-white, often pinkish-white and bluish-green, rarely brownish; markings more vague and unaccentuated; spots mainly reddish-brown tinged, rarely brown tinged, generally larger; underlying spots gray, generally more striking (Hoy and Ottow 1964). For comparison Bay-winged Cowbird eggs have ground color mainly grayish-white, often white and pinkish-white, rarely bluish-green; markings more sharply accentuated and more scattered; spots mainly reddish tinged, less frequently brownish tinged, mainly rather small; underlying spots gray, often less striking.

Comparing Screaming and Bay-winged Cowbird eggs, reddish or pinkish background colors predominate for Screaming Cowbird and white and gray background colors predominate for Bay-winged Cowbirds (Fraga 1983): size of marks are less reliable, marks tend to be more uniformly distributed for Screaming Cowbird; thick, dark lines ("scrawls") were more abundant on Screaming Cowbird eggs (145 of 246 with scrawls; only 11 of 229 Bay-winged Cowbirds had scrawls), a useful character to use for separation.

Surface texture. Slightly glossy.

Eggshell thickness. No information.

Clutch size. Bay-winged Cowbird nests usually receive 2 Screaming Cowbird eggs (Friedmann 1929: 49); in provinces of Salta and Jujuy, a parasitized nest would have 6-20 Screaming Cowbird eggs (Hoy and Ottow 1964); many parasitic eggs are evicted by Bay-winged Cowbird host, especially those laid before host begins incubation.

Egg laying. During morning (before 11:00-12:00 h; Hoy and Ottow 1964).


Onset of broodiness and incubation. No broodiness; does not incubate own eggs; incubation accomplished by host.

Incubation patch. No information.

Incubation period. 12-13 d (Sick 1985, 1993); 12 d (n=10; R. M. Fraga in Briskie and Sealy 1990); 12.0 d ± 0.1 SE (range 11-13; n=40 eggs; De Marisco et al. 2010). For comparison, the respective for the Bay-winged Cowbird are: 13.1 (range 13-14; n=7; R. M. Fraga in Briskie and Sealy 1990); 13.0 d ± 0.1 SE (range 12-14; n=42; De Marisco et al. 2010).

Parental behavior. Incubation provided by host species.

Hardiness of eggs against temperature stress; effect of egg neglect. No information.


Preliminary events. No information.

Shell breaking and emergence. Only 35 of 59 eggs in the nest at end of incubation hatched (Fraga 1986, 1998). For Bay-winged Cowbird, 78 of 86 eggs hatched (Fraga 1986, 1998).

Time of day: no information.


Condition at hatching. Altricial and nidicolous. At hatching skin has reddish color which becomes pink or pale pink, bill pinkish and shows no dark area around white eggtooth (in comparison, Bay-winged Cowbird skin becomes orange, bill is pinkish with darker pigmented area around white eggtooth; Fraga 1979, 1998). At hatching 2.4 g, skin orange pink inclined to be dusky, eye skin bluish green, feet pinkish yellow, bill pinkish yellow with gape white, mouth lining reddish, claws light yellowish, down dusky mouse gray, neossoptiles only on head, spinal, humeral and alar tracks (Friedmann 1929). At hatching, eyes closed and no visible feather tracts (Fraga 1986).
Growth and development. Skin hue [differences] remains same (between both cowbird species) until age 4 - 5 d (Fraga 1979). At 3 d, 6 g, bill lost pinkish tint, primaries and secondaries not yet break through skin, eyes closed; bird was dead next day (Friedmann 1929). Mean mass at day 0, 3.41 g ± 0.66 SD (n=15); at day 12, 52.5 g; growth rate, 0.438 g/d-g (Fraga 1986). Eyes begin to open about day 2 or 3; pinfeathers begin to open by day 7 or 8 and eyes are fully open (Fraga 1986). Growth constant (K) 0.503 and asymptotic mass 47.1 g (n=11; Fraga 1998); growth constant 0.51 ± 0.01 SE (n=8; Mermoz and Fernández 2003). Growth rate for males (n=4): growth constant 0.50 ± 0.02 SE, maximum growth rate 6.37 g/d ± 0.14 SE, asymptotic mass 51.6 g ± 1.9 SE; for females (n=6): growth constant 0.49 ± 0.02, maximum growth rate 5.23 g/d ± 0.22 SE, asymptotic mass 42.5 g ± 0.6 SE (De Marsico et al. 2010).

Behavior. No information.


Provided by host species.


Not known to occur.


Departure from the nest. Leave nest at 13.8 d (range 12-16, n=15; Fraga 1986). For comparison, Bay-winged Cowbird remain in nest 14-15 d (Fraga 1979).

Growth. No information once young leave nest.

Association with parents or other young. Dependent on host adults 3 wk after leaving nest (Fraga 1986).

Ability to get around, feed, and care for self. Nestlings younger than 13 d could not fly (usually) but could run and climb (Fraga 1986). Young seen taking caterpillar 51 d after leaving nest; feeding self about 5 weeks after leaving nest (Fraga 1986). Captive Screaming Cowbird was feeding self about 30 d after hatching (Fraga 1986). Daily post-fledging daily survival for 35 d was 0.969 (which may be low because of dispersal from study area; Fraga 1986).


No information.

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.