Screaming Cowbird

Open Bird Bucket List ↗
Screaming Cowbird

Quick Facts

  • Scientific Name: Molothrus rufoaxillaris
  • Spanish Name: Tordo chillón
  • Family: Icteridae
  • Known Nicknames: Screaming Cowbird
  • Average Length: 18–19 cm / 7.1–7.5 in
  • Average Weight: 40–55 g / 1.4–1.9 oz
  • Wingspan: 28–32 cm / 11–12.6 in
  • Key Feature: Iridescent black plumage with chestnut wing linings (rufoaxillaris refers to red axillaries)
  • Primary Diet: Primarily Granivore and Insectivore
  • Range: Central South America (eastern Bolivia, Paraguay, southern Brazil, Uruguay, northern Argentina)
  • Habitat: Open woodlands, savannas, grasslands, agricultural areas, and edges of marshes
  • Social Structure: Highly Social (often found in flocks, sometimes with other cowbirds)
  • Nesting/Breeding: Brood parasite; lays eggs in nests of other birds, primarily the Bay-winged Cowbird (Agelaioides badius)
  • Conservation Status: Least Concern (LC)
  • Population Trend: Stable

The Screaming Cowbird (*Molothrus rufoaxillaris*) is a fascinating brood parasite native to South America, renowned for its piercing, drawn-out whistle that gives it its name. This sleek, medium-sized bird displays a striking sexual dimorphism: males are a glossy, deep black with a subtle purplish sheen, while females are a more subdued sooty-brown. Its most unique special feature is its exclusive host relationship with the Bay-winged Cowbird—rather than parasitizing many species like other cowbirds, the Screaming Cowbird almost always lays its eggs in the nests of its larger, grayish cousin. This specialized strategy, combined with its loud, melodic calls and sharp, pointed bill, makes it a standout example of avian adaptation in the grasslands and open woodlands of Argentina, Bolivia, and Brazil.

Fun Facts

Despite its name, the Screaming Cowbird doesn’t actually scream; its call is a high-pitched, insect-like buzz, but it earned its moniker for its aggressive, piercing vocalizations during territorial disputes. It is a brood parasite, but with a twist: it almost exclusively targets the nests of other cowbird species, like the Shiny Cowbird, making it a “specialist parasite” that tricks its own relatives into raising its young. Its chicks have evolved to mimic the begging calls and even the skin coloration of the host’s offspring so precisely that the surrogate parents cannot tell them apart, ensuring they get fed first.

Habitats & Distribution

The Screaming Cowbird inhabits open and semi-open landscapes, primarily in lowland regions. It is strongly associated with areas of tall grasses, savannas, pastures, agricultural fields, and the edges of marshes or wetlands. It avoids dense forests, instead favoring habitats with scattered trees, shrubs, or fence lines that provide perching sites for its brood-parasitic behavior. This species is endemic to South America, with a distribution centered on the southern half of the continent. Its core range includes central and eastern Argentina, Uruguay, and southern Brazil (especially the state of Rio Grande do Sul), with more scattered populations extending into Paraguay and possibly southeastern Bolivia.

Behaviours & Reproduction

The Screaming Cowbird is a brood parasite, meaning it lays its eggs in the nests of other bird species, primarily the Bay-winged Cowbird. Unlike many brood parasites, it does not remove host eggs but relies on its chicks outcompeting or ejecting the host’s young. Mating is promiscuous, with no pair bonds formed; males court females with loud, screeching calls and visual displays, but females mate with multiple males and show no parental care. Socially, they gather in loose, noisy flocks outside the breeding season, often associating with other cowbird species. A unique reproductive strategy is their extreme host specialization—they almost exclusively parasitize Bay-winged Cowbirds, whose nests they locate by following host parents. The female Screaming Cowbird synchronizes her egg-laying with the host’s clutch, and her eggs are adapted to mimic the host’s egg color and pattern, reducing detection. This specialization reduces competition with other cowbird species and ensures high success rates, though the host often builds a new nest atop the parasitized one.

Diet

The Screaming Cowbird is an obligate brood parasite, meaning it does not raise its own young, but its diet consists primarily of seeds and insects. It forages on the ground, often in mixed-species flocks with other cowbirds and blackbirds, where it consumes grass seeds, weed seeds, and grains from agricultural fields. During the breeding season, it increases its intake of protein-rich insects such as beetles, caterpillars, grasshoppers, and spiders, which are crucial for egg production. An interesting fact about its diet is that it frequently follows grazing livestock, such as cattle, to catch insects flushed from the grass by the animals’ movement, a behavior known as “following.” Additionally, while its parasitic chicks are fed by foster parents—typically oropendolas, baywings, and other songbirds—the host parents often provide a different, more insect-heavy diet than the adult cowbird consumes itself.

Colors

The adult male Screaming Cowbird is glossy black overall, with a distinctive bright rufous-chestnut patch on the underwing coverts (visible in flight) and a slight purple-blue iridescence. Females are a plain, dull grayish-brown with a paler, faintly streaked underside, lacking the rufous patch. Juveniles are similarly drab but have buffy wingbars and a mottled appearance. There is no camouflage adaptation; the male’s high-contrast black and rufous pattern is likely used for social signaling rather than concealment.

Distribution Map

Places linked to this species. A sighting is never guaranteed.