Screaming Cowbird

Is a medium-sized passerine bird measuring between 7 to 8.2 inches (18 to 21 centimeters) in length. The adult plumage is predominantly silky black with intense blue-violet iridescence when illuminated by direct sunlight, which is slightly duller in females. It features a characteristically short, thick, and conical bill, entirely black, which is highly adapted for a generalist diet. A diagnostic anatomical feature, though typically concealed when perched, is the presence of rufous-cinnamon underwing coverts, a trait that inspires its scientific name. Juveniles exhibit an extraordinary case of mimicry, possessing a grayish-brown plumage with cinnamon wing patches that renders them almost indistinguishable from the chicks of their primary host species.

Geographical distribution: This species is widely distributed across the Southern Cone of South America. Its geographic range encompasses northern and central Argentina, the entirety of Uruguay, central and southern Paraguay, southeastern Bolivia, and the southern regions of Brazil, particularly within the state of Rio Grande do Sul. In recent decades, its range has expanded significantly northward into southeastern Brazilian states, driven by deforestation and landscape modification.

Habitat: Primarily frequents open and semi-open landscapes. It is commonly found in humid grasslands, shrubby savannas, open xerophytic woodlands, agricultural pastures, crop fields, and suburban parks. It strictly avoids dense, continuous forest interiors, displaying a strong preference for forest edges and anthropogenic environments where food resources and host nests are abundant.

Feeding: Maintains an omnivorous diet with a strong emphasis on seeds and terrestrial invertebrates. It forages extensively on the ground, walking deliberately to consume grass seeds, agricultural grains, and a wide array of insects such as beetles, grasshoppers, ants, and caterpillars. It frequently associates with grazing cattle, capitalizing on the insects flushed out of the grass by the movement of the livestock.

Behavior: Highly gregarious in nature, this species regularly forms large flocks outside of the breeding season, often mixing with other icterids like Molothrus bonariensis. Males perform elaborate courtship displays that involve puffing up their neck feathers, drooping their wings, and uttering a series of metallic, squeaking, and buzzing vocalizations. Its flight pattern is strong, direct, and slightly undulating over long distances.

Nesting: Exhibits an obligate brood parasitic lifestyle with a high degree of host specialization. Its primary host is the Grayish Baywing (Agelaioides badius). Females do not build nests or incubate eggs; instead, they deposit one or two speckled eggs into the host´s nest, often puncturing or ejecting the host´s own eggs to secure space. The parasitic chicks display an astonishing acoustic and morphological mimicry of the host’s offspring, ensuring they receive equal feeding rates from the foster parents.

Conservation status: Categorized as Least Concern (LC) on the IUCN Red List. Population trends are stable and growing, as the expansion of cattle ranching and agricultural activities continues to create suitable open habitats while facilitating the dispersal of its primary host species.