Screaming cowbird

The screaming cowbird (Molothrus rufoaxillaris) is an obligate brood parasite belonging to the family Icteridae and is found in South America. It is also known commonly as the short billed cowbird.

The screaming cowbird has mildly iridescent black plumage; the lesser under-wing coverts are rufous. The female is slightly duller in colour than the male. The legs are black and the iris is reddish brown. Adult body length is 18–21 cm and mean adult weight is 58 g for males and 48 g for females. The call of the screaming cowbird was first described as "impetuous screaming notes"; however, a more useful description for field identification is noisy, explosive and piercing with rasp-like calls also produced. Screaming cowbirds are mostly seen in pairs or small flocks.

Within South America, the screaming cowbird is found in north east and central Argentina, south east Bolivia, central Brazil and throughout Paraguay and Uruguay. Its natural habitat is pastureland where it forages amongst grazing animals such as cows, hence the name "cowbird". Similar to other cowbirds, it forages predominantly on the ground, eating invertebrates that have been disturbed by grazing stock. The distribution of the screaming cowbird has increased significantly in recent decades due to habitat alteration caused by deforestation and by following its hosts into new areas. Both female and male screaming cowbirds show high fidelity in roosting locations.

The screaming cowbird is a specialist brood parasite, predominantly parasitizing the nests of baywings (Agelaioides). In 1874, W H Hudson was first to observe this parasitic relationship when he witnessed what he believed to be baywing chicks morph into screaming cowbird plumage.

The screaming cowbird also parasitizes the nests of the chopi blackbird (Gnorimopsar chopi) and the brown-and-yellow marshbird (Pseudoleistes virescens). Parasitism of these other two species generally occurs in areas where baywings are absent but can also occur in its presence. The three host species that can successfully raise screaming cowbirds are all cooperative breeders. "Helpers" at the nest provide assistance with chick feeding and with predator defense.

Screaming cowbirds are monogamous and form stable pairs for the duration of the breeding season. As obligate brood parasites, they do not build their own nests, and instead, parasitize the nests of other species, predominantly baywings.

Most baywing nests are parasitized by the screaming cowbird with parasitism rates of 74-100% recorded. Parasitism rates of 5-20% have been recorded for the brown and yellow marsh bird and 46% for the Chopi blackbird.

Screaming cowbird eggs are spotted like those of their main host, the baywing, but do vary in shape, background colour and markings. Although this may be obvious when they appear in the host nest during pre-laying, they can be difficult to detect in a nest full of eggs.