English               -               Screaming Cowbird

Portuguese       -                Vira-bosta-picumã

Latin                  -                 Molothrus rufoaxillaris

The Screaming Cowbird is an obligate brood parasite belonging to the Icteridae family of birds. It is also known commonly as the short billed cowbird.      

The Screaming Cowbird has a 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 around 21 cm in length and weighs up to 58 g. 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. Screaming Cowbirds are mostly seen in pairs or small flocks.

The Screaming Cowbird is found in South America. It is found In parts of Argentina, Bolivia, Brazil and  throughout Paraguay and Uruguay. Its natural habitat is pastureland  but can also can be found in open areas in cities, parks, gardens, etc.

Similar to other species of cowbirds, the Screaming Cowbird forages predominantly on the ground, eating invertebrates such as spiders and a variety of insects that have been disturbed by grazing stock. where it forages amongst grazing animals such as cows, hence the name “cowbird”. 

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, parasitise the nests of other species, predominantly the Bay-winged Cowbird. Most Bay-winged Cowbird nests are parasitised by the Screaming Cowbird. The Screaming Cowbird is also known to  parasitise the nests of Brown and Yellow Marsh bird and the Chopi Blackbird. Screaming cowbird eggs are spotted like those of their main host, the Bay-winged Cowbird, 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. Screaming Cowbirds can lay 6-20 eggs in a Bay-winged Cowbird nest but usually one pair will lay only 2 eggs in the host nest. Up to 12 female Screaming Cowbirds can parasitise the same Bay-wing Cowbirds nest. Screaming Cowbird adults frequently pierce the eggs of their hosts as well as previously laid parasite eggs. Screaming Cowbirds can distinguish between their eggs and those of other species. The purposes of egg puncture behaviour are not clearly understood but one reason could be to reduce nestling competition and to enhance survival of the Screaming Cowbirds offspring.  Screaming cowbirds nestlings deceive their main host with superb visual chick mimicry and calling, in addition, they can beg for longer and at a higher intensity. This more intense begging does not reflect greater hunger demands; instead, reflects a hard-wired behaviour to ensure adequate nourishment and survival. They are the only avian brood parasite to exhibit this trait. There are slight differences in skin and bill colour of nestling Screaming Cowbirds and the host nestlings but this is only obvious for the first 4–5 days. They then remain almost identical in size and appearance until they become nutritionally independent.

The global population size has not been quantified, but this species is described as ‘fairly common’. 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 ranges. It is considered that the Screaming Cowbird is a species “of least concern”.

Photos of the Screaming Cowbird were taken at Parque Barigui - Curitiba - Parana.