Usually on ground (Skutch 1954, 1996). Terrestrial foraging, foliage searching and foraging upon mammals (Robinson 1988). Much foraging on ground, sometimes in association with cattle; also will search for insects on backs of cattle or mules (Skutch 1954, 1996). Observed eating bananas in plantations (Goodfellow 1901) and corn (Sclater and Salvin 1873). When foraging along rivers, their "chief occupation" was stone-turning, their "head was lowered and the tip of the bill inserted beneath the near edge of the stone and shoved forward, in the line of advance of the bird. As the push was given, the lower mandible was dropped somewhat and the bill held slightly open" (Skutch 1954).
Mating system unknown, described as promiscuous (Friedmann 1929, Ortega 1998).
Courtship displays. Males approach female with a stiff, "goose-step" walk, with chest outswollen and head drawn back; when near, male would draw himself up, arching neck and depressing head until bill rests on out-puffed plumage of breast, cape feathers erected; may fluff plumage and bob up and down by flexing legs (Skutch 1954; see also figure in Sick 1985, 1993). Courtship display described by Friedmann (1929: 357) as follows: "Male elevates his nuchal cape and bends his head forward and down so that the bill rests for its entire length against the feathers of the breast. The eyes seem to be brought forward because the feathers of the cape and hind neck give the head a very wide, owl-like appearance. The body feathers are fluffed out and the tail drawn slightly under and forward. The bird does not bow forward as do the Cowbirds [i.e., Molothrus spp., sensu strictu]." [In 1929, Friedmann's definition of Molothrus included Screaming Cowbird M. rufoaxillaris, Brown-headed Cowbird M. ater and Shiny Cowbird M. bonariensis.] Solicitation display of female not described.
Social and interspecific behavior
Degree of sociality. Flocks.
Nonpredatory interspecific Interactions. Observed foraging for insects (biting horse flies Tabanidae) on backs of capybaras (Hydrochoerus hydrochoerus) and on back of tapir (Tapirus terrestris; Robinson 1988), and on cattle, mules and swine (von Pelzeln 1871, Goodfellow 1901, Skutch 1996). In Costa Rica, sometimes forage together with Bronzed Cowbirds (Molothrus aeneus) and Groove-billed Anis (Crotophaga sulcirostris), in pastures with grazing cattle, capturing disturbed insects (Skutch 1954, 1996). Cowbirds foraged in large flocks in French Guiana with Smooth-billed Ani (Crotophaga ani) and Yellow-rumped (Cacicus cela) and Red-rumped (Cacicus haemorrhous) caciques (Dick et al. 1984).
Head-Down (Rothstein 1980; called Interspecific Preening Invitation by Selander and La Rue 1961): Bird approaches another and assumes posture with head down, bill to breast and nape feathers raised (Harrison 1963, Payne 1969). Chapman (1928) saw female Giant Cowbird display towards a female Chestnut-headed Oropendola (Psarocolius wagleri) at oropendola's nest. Individual males in zoos displayed towards African Black-headed Lapwing (Vanellus tectus; Harrison 1963) or to humans (Payne 1969). Similar to same display of other cowbirds (Brown-headed Cowbird Molothrus ater, Rothstein 1980; Shiny Cowbird M. bonariensis, Selander 1964; Bronzed Cowbird M. aeneus, Selander and La Rue 1961).
Males rarely seen near host colonies (Wetmore et al. 1984). Two potential host species in Peru, the Russet-backed Oropendola (Psarocolius angustifrons) and Yellow-rumped Cacique (Cacicus cela) defended their colonies against cowbird parasitism; Giant Cowbirds concentrated most of their visits on oropendola nests, which were sometimes left untended when the colony members were foraging together in a flock away from the colony. Yellow-rumped Caciques, however, seldom left their colonies untended and no cowbirds were known to fledge from cacique nests during the five years of the study. Robinson (1988) reported that when Giant Cowbirds visited host colony in groups, cowbirds' visit seemed coordinated – male or 1 female may "lure" [= be chased by] oropendolas away from their nests and other females take opportunity to look into nests. Cowbirds approaching nests of Montezuma Oropendolas (Psarocolius montezuma) were attacked and driven from colony; cowbirds able to enter nests during only 7 of 83 visits; late morning visits were most successful for female cowbirds to enter nests (Webster 1994). Pair of cowbirds attacked by both sexes of Olive Oropendola (Psarocolius bifasciatus) attacked a pair of cowbirds that landed near a cluster of oropendola nests (Fraga and Kreft 2007).
Vigorously chased by Green Jay (Cyanocorax yncas) during breeding season, but ignored by this species at other times (Alvarez 1975, Gayou 1996).
Predators of adults likely to be similar as those of other forest birds.