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 main host, the baywing, can successfully fledge 1 screaming cowbird for 3 of its own. Reproductive success, as the number of fledgling per egg laid, has been recorded to be 0.14 for the screaming cowbird when hosted by the baywing. When hosted by the chopi blackbird, a reproductive success rate of 0.17 was found In addition, the brown-and-yellow marshbird is also able to successfully rear screaming cowbird chicks.
The main host species, the baywing, clearly suffers losses through intense parasitism by the screaming cowbird; however, they are able to successfully raise their young with little overall impact in terms of hatching success, survival of nestlings and fledgling body mass.
