Screaming Cowbird

The Screaming Cowbird, known scientifically as Molothrus rufoaxillaris, is a bird cloaked in mildly iridescent black plumage, with the lesser under-wing coverts displaying a rufous hue. The males are slightly more vibrant than the females. These birds possess black legs and a reddish-brown iris, with adults measuring between 18 to 21 centimeters in length. Males typically weigh around 58 grams, while females are lighter at approximately 48 grams.

Identification Tips

When attempting to identify the Screaming Cowbird, look for its noisy, explosive, and piercing calls, which may also include a rasp-like quality. These birds are often observed in pairs or small flocks, and their distinctive vocalizations can be a reliable indicator of their presence.

Habitat

The Screaming Cowbird is a denizen of pasturelands, often seen foraging among grazing animals such as cows, which is the origin of its common name. It is adapted to ground foraging, seeking out invertebrates that have been disturbed by the movement of livestock.

Distribution

This species is native to South America, with its range encompassing northeast and central Argentina, southeast Bolivia, central Brazil, and throughout Paraguay and Uruguay. The Screaming Cowbird's distribution has expanded in recent decades due to habitat changes from deforestation and by following its hosts into new areas.

Behaviour

Screaming Cowbirds exhibit high fidelity to their roosting locations. They are monogamous, forming stable pairs throughout the breeding season. As obligate brood parasites, they do not construct their own nests but instead lay their eggs in the nests of other bird species, primarily the baywing.

Song & Calls

The Screaming Cowbird's vocalizations were initially described as "impetuous screaming notes." For field identification, listen for their characteristic noisy and explosive calls, which may also include a rasp-like sound.

Breeding

Host Species

The Screaming Cowbird is a specialist brood parasite, mainly targeting the nests of baywings (Agelaioides). It has also been known to parasitize the nests of the chopi blackbird (Gnorimopsar chopi) and the brown-and-yellow marshbird (Pseudoleistes virescens), especially in areas where baywings are absent.

Parasite Behaviour

Screaming Cowbirds lay eggs that are spotted similarly to those of their main host, the baywing, although there can be variations in shape, background color, and markings. They can lay multiple eggs in a single baywing nest, with up to 12 females parasitizing the same nest. Adults of this species have been observed piercing the eggs of their hosts and other parasite eggs, possibly to reduce competition and enhance the survival of their offspring.

Host Response

Baywings have been known to eject Screaming Cowbird eggs, particularly during the pre-laying period, reducing the parasitic egg load significantly. However, once hatched, baywings treat parasitic chicks as their own, providing food, protection, and even removing ectoparasites.

Reproductive Success

The baywing can successfully fledge one Screaming Cowbird for every three of its own chicks. The reproductive success of the Screaming Cowbird varies depending on the host species, with recorded success rates of 0.14 when hosted by baywings and higher rates with other hosts.

Similar Species

There are no similar species mentioned in the provided content.

Diet and Feeding

Screaming Cowbirds forage predominantly on the ground, consuming invertebrates that have been disturbed by grazing animals.

Conservation Status

The Screaming Cowbird is currently listed as Least Concern on the IUCN Red List, indicating that it is not at immediate risk of population decline or extinction.