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Agelaioides badius

Bay-winged Cowbird

  • Order: Passeriformes
  • Family: Icteridae
  • Polytypic 3 Subspecies

Authors: Lowther, Peter E

Life History


Main food items reported for nestlings are grasshoppers (Acrididae and Tettigoniidae), mantids (Mantidae), green stinkbugs (Pentatomidae) and caterpillars (particularly Hesperiidae); older fledglings and adults also eat seeds (grasses and Asteraceae; Fraga 1986). Include distinctive caterpillars of Epargyreus tmolus (Fraga 1991). Friedmann (1929) gave following list of gizzard contents: weed seeds, corn, rice, locust remains, coleopteran and lepidopteran larvae, flies, longicorn beetles, and small snail.


Preening, head-scratching, stretching, bathing, anting, etc. Usually forages on the ground, often under cover. 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 information.

Physical interactions. No information.
Communicative interactions. A collective agonistic display, Leaf Gathering, occurs throughout year; display includes singing by most interacting individuals, birds hold bits of vegetation (leaves, bark or twigs) in bill, assume Bill Tilt (Bill Up or Head Up) posture; fighting or supplanting may occur, yearlings stay at edge of group in "submissive postures" (Fraga 1991). Leaf Gathering displays involve 8 - 26 birds, last 4.3 - 21 min, and occur on ground or in trees; 7 of 22 known contexts involved attendants of 1 nest responding to a traveling group with fledglings (Fraga 1991).
Bill Tilt given with head back and bill up, to bill-head-neck-body vertical with feathers sleeked; directed to other birds, usually from dominant to subordinate. Display similar to that of Molothrus cowbirds but "involving a lesser extension of the neck" (Selander 1964). Also called Bill Point or Head-up in Molothrus cowbirds (see Lowther 1993).


Little information on territoriality.  Nearest neighbor distances between nests at peaks of 2 breeding seasons was about 46 m (range 25 - 103; n = 42; Fraga 1991).

Sexual Behavior

Mating system and sex ratio: Apparently monogamous; of 40 observed copulations of marked birds, only 1 female seen copulating with 2 males, both males subsequently provisioned nest (Fraga 1991).
Pair bond. Most pairs split between breeding seasons, no pair lasting more than 2 seasons (Fraga 1991).
Courtship displays. Friedmann (1929: 5) noted "no courtship display of any kind," a statement with which Selander (1964) agreed. Solicitation (Pre-copulatory) display of female not specifically described.
Extra-pair copulations. No information.

Social and interspecific behavior

Degree of sociality. Considered "extremely" social; usually found in small flocks of 3 - >20 individuals; "winter" [=non-breeding season] groups average 21 birds; flocks in "summer" are smaller; single birds and pairs observed near nest during early stages of nesting (Fraga 1991; Jaramillo and Burke 1999; see also Lamm 1948). Cooperative breeders (Fraga 1986).
Play. No information.
Nonpredatory interspecific interactions. Bay-winged Cowbirds may solicit preening by assuming "Allopreening Invitation Posture" (see Selander 1964). [In Molothrus cowbirds, this "Preening Invitation" display has been called the "Head Down" display; the "Preening Invitation" display in Bay-winged Cowbirds does not have similar posture and, considering taxonomic studies, this display may not be homologous.] This display begins with the bird crouched and sidling toward recipient; the body held horizontal with plumage compressed, bill pointing downward and tail flicking down and up; at near approach, tail-flicking stops, neck pulled in and bill raised to 2oo above horizontal; if approached by recipient, fluff feathers of head and neck and angle bill further upward to 80o; description based on captive male displaying to male Chestnut-capped Blackbird (Chrysomus ruficapillus; Selander 1964).
May share roost site with Screaming Cowbirds; roosts without Screaming Cowbirds in wooded areas, for 30 mixed flocks, counts of both species totaled 396 Bay-winged and 85 Screaming cowbirds (Fraga 1998).
Will attack Screaming and Shining cowbirds that visit and may parasitize nests (Fraga 1986). Mounted pairs of Screaming Cowbirds could not be placed within 15 m of a Bay-winged Cowbird nest without Bay-winged Cowbird attacking the mounts (Fraga 1986). Of 141 nest visits by Screaming Cowbird, 32 (of 34) attacks elicited when female Screaming Cowbird approaches nest, otherwise visits ignored; attacks sometimes with physical contact, most often chases or supplanting (Fraga 1998).
In contrast with Molothrus cowbirds, the Bay-winged Cowbird never is seen following grazing mammals (Fraga 1998).


Kinds of predators: Predators of adults likely to be similar as those of other small birds. Remains of Bay-winged Cowbird, probably scavenged, found in toad Ceratophrys ornata (Friedmann 1929).
Response to predators: Adult and helper Bay-winged Cowbirds mob or scold potential predators near nest: cat Felis geoffroyi and human investigator (Fraga 1986). Screaming Cowbirds (which visit Bay-winged Cowbird nests at rates up to 9 visits/h) were most often attacked; also attacked were Chimango Caracara (Milvago chimango) and Roadside Hawk (Buteo magnirostris; Fraga 1992). During 12 days of observation, Aplomado Falcons (Falco femoralis) made 34 attacks on mixed flocks of Bay-winged and Screaming cowbirds; Bay-winged Cowbirds gave hawk-alarm calls that immobilized the whole flock; Screaming Cowbirds remained silent; both species responded similarly also to "other predators" (Fraga 1986, 1998).



Pair formation: No information.

Nest building: No information.

First/only brood per season. Breeding season during a "dry and warm" year was 2 weeks later than the preceding year with "much and early rainfall" (Hoy and Ottow 1964).

Egg dates: November to early March (Fraga 1986); 27 October - 10 March (Fraga 1988, 1998).


Selection Process: Females using nestboxes had used boxes as roost sites (Fraga 1986).

Microhabitat: Nest sites varied, built in vines, palm trees and in cavities; cavity nests, including covered structures built by other birds (especially nests of furnariids) as well as nest boxes and other artificial sites (Fraga 1986). Sites in foliage included among fronds of palms (Washingtonia robusta, Trachycarpus fortunei and Phoenix canariensis); in foliage of conifers (Araucaria bidwillii and Cupressus sempervirens); in ivy (Hedera helix); and in an epiphytic bromeliad (Tillandsia aeranthos; Fraga 1988). Cavity sites (including woodpecker cavities) in trees and palms were most common natural nest sites (Fraga 1988). Use old closed or domed nests of thornbirds (Phacellodomus spp.), spinetails (Synallaxis spp.), Rufous Horneros (Furnarius rufus), Firewood-gatherers (Anumbius annumbi), and Great Kiskadees (Pitangus sulphuratus; Friedmann 1929). Nested in nests of Brown Cachalote (Pseudoseisura lophotes (which build domed nest with tubular entrance that lasts several years), in abandoned holes of Green-barred Woodpecker (Colaptes melanochloros) in the cactus Trichocereus terscheckii, or in crevices at base of palm fronds (Orians et al. 1977). Bay-winged Cowbirds usually builds their own nests or make use of old nests of other species (e.g., nests of Anumbius, Pseudoseisura or Monk Parakeet Myopsitta), but rarely pirates nests (e.g., Little Thornbird [Phacellodomus sibilatrix]). Nests of others species that have been appropriated include Rufous-fronted Thornbird (Phacellodomus rufifrons), Rufous Hornero (Furnarius rufus) and occasionally Short-billed Canastero (Asthenes baeri) or other species (once in nest of Green-barred Woodpecker [Colaptes melanochlorus]; Hoy and Ottow 1964). Of 28 domed nests of twigs of Firewood-Gatherer (Anumbius annumbi), 16 were used by Bay-winged Cowbirds (Fraga 1988). Rufous Horneros build new nests each breeding season, hornero nests used by Bay-winged Cowbirds had been built 3 and 4 years prior to cowbird use (Fraga 1988). Heights of nests used ranged from 1.3 - 12 m (De Mársico and Reboreda 2008).  Among 85 breeding attempts, 35 used nests of Little Thornbird, 33 in nests of Greater Thornbird (Phacellodomus ruber), 4 in nests of Lark-like Brushrunner (Coryphistera alaudina), 4 in nests of Golden-winged Cacique (Cacicus chrysopterus), 3 in natural cavities and remainder in nests of Rufous Hornero, Chotoy Spinetail (Schoeniophylax phryganophila), Cattle Tyrant (Machetornis rixosa), Solitary Cacique (Cacicus solitarius) and cavity of Campo Flicker (Colaptes campestris) (Di Giacomo and Reboreda 2015).


Construction process: Will make use of domed nests of other species (e.g., furnariids, Great Kiskadee [Pitangus sulphuratus]; Fraga 1986). Males may begin nest building, but female brings some lining before starting clutch (Fraga 1986, 1992). Bay-winged Cowbirds build nest of grass and fibers within structure or cavity being used (Fraga 1988). Most nest material obtained from within 30 m of nest (Fraga 1992). There usually is an interval between the end of any sustained nest building activity and the first egg ("pre-laying interval"): 6.2 d ± 2.3 SD (range 3 - 12, n = 21; Fraga 1986).

Structure and composition matter: Nest shell of grasses or other fibers (Fraga 1992).
Dimensions. Cavities were 15 - 25 cm deep, open at top or with more than 1 entrance (Fraga 1988).

Microclimate: No information.

Maintenance or reuse of nests, alternate nests: Males known to use successful nest sites up to 6 breeding seasons, females for 2 seasons (Fraga 1988). Successful nestboxes were more often reused (5 of 6 reused during second year) than unsuccessful nestboxes (1 of 5; Fraga 1988).

Non-breeding nests: No information.


Shape: Usually ovate, not as round as Screaming Cowbird.

Size: Mean egg size: 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, 1998; see also Briskie and Sealy 1990). [For comparison, mean size of Screaming Cowbird eggs: 23.184 mm ± 0.966 SD (range 20.05 - 25.80; n = 255) x 17.842 ± 0.644 SD (range 15.55 - 19.35; n = 225; 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: 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). [For comparison, mean size of Screaming Cowbird eggs: 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).]

Relative shell thickness, q = LB/m, where L = length, B = breadth, m = shell mass in mg (see Rey 1892), is 1.62 (range 1.49 - 1.79) [and for comparison, mean relative shell thickness of Screaming Cowbird eggs: 1.28 (range 1.15 - 1.50)] (Hoy and Ottow 1964).

Mean size: 23.6 x 18.6 mm (n = 36) (Mason 1985; [For comparison, mean size of Screaming Cowbird eggs 23.2 x 17.6 mm (n =10)]).

Mass: Mean mass 4.03 g ± 0.33 SD (range 3.20 - 4.80; n = 113; Fraga 1983). [For comparison, mean mass of Screaming Cowbird egg, 4.04 g ± 0.38 SD (range 3.00 - 5.20, n = 117; Fraga 1983).] Mean 4.5 g (n = 10; Mason 1985. [For comparison; mean mass of Screaming Cowbird eggs, 4.2 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 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 (Hoy and Ottow 1964) (For comparison, Screaming Cowbird eggs 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).]

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: 0.127 mm (Schönwetter 1981, see also Rahn et al 1988).

Clutch size: Mean clutch size 3.8 eggs ± 0.7 SE (range 3 - 5, n = 62; De Mársico and Reboreda 2008). For unparasitized clutches, 3.75 eggs ± 0.50 SD (n = 4). 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: Little information. Eggs laid at “around 07:00", period between completion of nest and laying of first egg 6.4 d (range 1 - 19; n = 44); this interval tended to increase during the breeding season (De Mársico and Reboreda 2008).


Onset of broodiness and incubation: No information.

Incubation patch: No information.

Incubation period: Known incubation 13 or rarely 14 d (Fraga 1986); 13.1 d ± 0.7 SE (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 by female (Fraga 1986). Male seen to feed female 3 times during 2574 min of observation at 14 nests (Fraga 1992). Night checks at 14 nests showed only females present (Fraga 1992).

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


Preliminary events: No information.

Shell breaking and emergence: 78 of 86 eggs hatched (Fraga 1986, 1998). [For Screaming Cowbird, 35 of 59 eggs in the nest at end of incubation hatched (Fraga 1986, 1998).]

Time of day: no information.


Condition at hatching: Altricial (nearly naked and helpless) and nidicolous (confined to nest). At hatching, eyes closed and no visible feather tracts (Fraga 1986); skin color yellowish orange, bill is pinkish with darker pigmented area around white eggtooth (for comparison, Screaming Cowbird young: skin color reddish which becomes pink or pale pink, bill pinkish and shows no dark area around white eggtooth; Fraga 1979, 1998).

Growth and development: Friedmann (1929): Egg hatched at 07:00 h, nestling rested a few min, then faint peep; when dry, 2.3 g; skin dusky orange pink, eye skin dusky-bluish green; bill pinkish orange and gape white; feet orange pink with light yellow claws; down mouse-gray; inside of mouth reddish; down present on head, spinal, humeral, alar and femoral tracts; neossoptiles about 10 mm on supraorbital tract, 4 mm on supraoccipital. Young probably 5 d old: 20.2 g; eyes open; feathers open on all tracts except head and uropygium, and abdominal and crural tracts which were still sheathed. Mean mass at day 0, 3.55 g ± 0.42 SD (n = 29); at day 12, 35.0 g; growth rate 0.431 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) 0.485 and asymptotic mass 34.9 g (n = 33; Fraga 1998).  Growth constant 0.47 ± 0.01 SE, maximum growth rate 4.25 g/d ± 0.07 SE, asymptotic mass 36.7 g ± 0.4 SE (n =
69 young in 25 broods; de Marsico et al. 2010).

Behavior: Little information.


Brooding: Observations during days 1 - 3 of nestling period, females brooded for 28 - 54% of observation period (Fraga 1992). Daytime brooding ceased after day 7 (Fraga 1992).

Feeding: Fed by parents and helpers (Fraga 1986). Provisioning rates, based on 3432 min observation at 13 nests covering days 1 - 12 of nestling period: males, 4.41 trips/h ± 2.17 SD (range 2.33 - 7.44, n = 13); females, 4.53 trips/h ± 7.48 SD (range 1.64 - 10.01, n = 13); helpers, 5.16 trips/h ± 12.27 SD (range 1.66 - 18.20, n = 12; Fraga 1992).

Nest sanitation: Both parents preened nestlings (Fraga 1992).


Helpers: No certain evidence that A. b. fringillarius form is cooperative breeder (R. Otoch in Jaramillo and Burke 1999); other populations show cooperative breeding.
Helpers were at least 10 mon old; most (12 of 17) marked juveniles remaining in study area helped; males predominated among known sex helpers (9 males, 1 female, 18 unknown); 2 males still helped in third year (Fraga 1991). Of 12 helpers marked as nestlings, 5 helped at nest of male parent, 2 at nest of female parent and 2 at nest of both parents (Fraga 1991). Helpers recruited during nestling period or later (Fraga 1991). Most nests (19 of 20) have 1 or 2 helpers (mean = 1.5, mode = 1; rarely, 0, 3 or 4) after eggs hatch; helpers feed nestlings, mob potential predators (Fraga 1972, 1986, 1991; see also Orians et al. 1977). Number of helpers increase from 0.66 helpers per nest at day 1 to 2.5 at day 12 (Fraga 1991). During nestling period, females seen to solicit food from other adults; the 4 copulations seen (during 3247 min observation) were with helpers (2 helpers subsequently mated with female; Fraga 1991).
Helpers fed young; guard nest; mob ... number of helpers accumulate through nestling period and more, seeming to gradually change into non-breeding flock

Results of helping. See Parental Care, Feeding above. [for parents, for nestlings, for helpers, variation in occurrence].


Identity of parasitic species: Screaming Cowbird and Shiny Cowbird.

Frequency of occurrence: Bay-winged Cowbird is parasitized by Screaming Cowbird (a brood parasite almost completely specializing on the Bay-winged Cowbird) and Shiny Cowbird (a wide-spread, generalist brood parasite). Interspecific breeding dynamics and intra-nest competition make for complicated interactions: eggs appearing in a nest before Bay-winged Cowbird egg laying are ejected; both parasitic cowbirds may puncture or eject eggs from the nest during their visits; parasitic cowbirds have overlapping home ranges; Bay-winged Cowbirds are cooperative breeders. One brood seen comprised of 3 Bay-winged Cowbirds, 1 Screaming Cowbird and 1 Shiny Cowbird (Fraga 1986)!  

Most nesting attempts 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); overall at 4 Argentina sites, 86 of 97 nests parasitized, with an average of 3.09 eggs/nest (Fraga 1998); 13 of 14 nests (Hoy and Ottow 1964), 15 of 15 nests (Mason 1980), all of 52 nests (Jaramillo 1993), and 181 of 193 nests parasitized (of these 155 by Screaming Cowbirds only, 1 by Shiny Cowbird only and 25 by both Screaming and Shiny cowbirds; de Marsico et al. 2010). In addition, most nests are multiply parasitized by Screaming Cowbirds with 1 to 12 (Fraga 1986), 15 (Hoy and Ottow 1964) or 19 eggs (Mason 1980).
Shiny Cowbirds are generalists in host selection and parasitize many other host species. At one study site, 19 Bay-winged Cowbirds out of 85 total nests were parasitized by Shiny Cowbirds and received 20 eggs of which only 3 hatched and no young survived more than 2 d after fledging; breeding season for both species overlapped for only 69 of the 85 nests; 13 of the 19 nests parasitized by Shiny Cowbirds were also parasitized by Screaming Cowbirds (Fraga 1986). Shiny Cowbirds regularly puncture or remove host eggs (Fraga 1986).

Response to parasitic mother, eggs, or nestlings: 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).  Egg ejection is accomplished by kicking; parasitic eggs are ejected if laid prior to Bay-winged Cowbird’s first egg and entire mixed clutches are ejected if heavily parasitized (De Mársico et al. 2013).  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 were eventually ejected (Fraga 1986). Parasitic eggs laid during the host's laying period are usually 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).

The disjunct (and possibly specifically distinct) population fringillarius of northern Brazil known to be parasitized only by Shiny Cowbird; Screaming Cowbirds first reported in the area in 1993 but have only parasitized Chopi Blackbirds (Fraga and D'Angelo Neto 2014). 


Departure from the nest: Leave nest at 14.0 d (range 12 - 16, n = 68; Fraga 1986). Adults may try to induce young to leave nest at 12 - 13 d; adult approach nestling with food in bill and then move backwards (Fraga 1986). Young and attendants leave nesting territory within 72 h (Fraga 1991). For one brood, 2 Bay-winged Cowbird young moved 80 m from nest on first day from nest (Shiny Cowbird young nestmate move only 25 m; Fraga 1986).

Growth: Little information once young leave nest. Red palate of nestlings change to black by 8 mon (Fraga 1991).

Association with parents or other young: Dependent on adults for 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). Banded young seen taking grass grains 28 d after leaving nest; observed taking caterpillars at this same age; feeding self 5 wk after leaving nest (Fraga 1986). Daily post-fledging daily survival for 35 d was 0.987 (Fraga 1986).


Little information: Young frequently solicit preening from adult or from other young with "head-up" posture [Selander 1964; see Social and interspecific behavior, nonpredatory interspecific interactions, above) and may engage in mutual allopreening (Fraga 1998). If threatened by adult, young withdraw head in submissive posture (Fraga 1998).

Populations and Demography

No information.

Recommended Citation

Lowther, Peter E. 2013. Bay-winged Cowbird (Agelaioides badius), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: