Bronzed Cowbird (Molothrus aeneus)

Distribution

Year-round range: Extreme southeastern California, southern Arizona, New Mexico, and Texas, and locally in southern Louisiana, extending south through central Mexico to Panama. The eastern and western are two distinct populations which overlap in western Texas.

Breeding habitat

Open country with occasional tree patches or large tall shrubs, such as in riparian woodlands. Often found in humid, hot climates, commonly in areas of human habitation as well as where cattle are grazed. In Mexico, often found in tropical evergreen forest, tropical deciduous forest and tropical and subtropical scrub.

Description

Male: Similar in shape to Brown-headed Cowbirds, but stockier, shorter tailed, and shows a thick ruff; also is smaller and shorter-tailed than blackbirds and grackles and has a more conical bill. Plumage is black and iridescent with glossy blue wings; the red iris and unique ruff on neck are distinctive.

Female: Birds in the eastern part of the range are similar to adult male, but duller and lack the glossy blue wings. Western birds are pale gray-brown, similar to female Brown-headed Cowbirds, but still with a red eye.

Juvenile: Pale gray-brown plumage that is darker on the upperparts, similar to the female Brown-headed Cowbird. Note the juvenile’s dull-brown eye color as compared to the adult’s distinctive red eye.

Vocalizations

Songs: A soft, tinny, rising series of wheezy and guttural notes with various squeaks and squeals.

Calls: “Whistle call” is a series of tinny, wheezy whistles and grating trills given by the male. Throughout the range there are three distinct dialects, possibly a fourth in western Arizona, with distinctive introductory notes.

Foraging strategy

Walks on the ground, frequently in association in cattle. Along with gleaning insects from their skin, captures insects stirred up by the animals. Bronzed Cowbirds commonly feed and roost in large flocks and are frequent visitors to bird feeders.

Diet

Insects from the ground, grain, and weed, grass, and grain seeds.

Behavior and displays

Bill-tilt: Used by males, rarely females, to defend area or to displace an individual. It is usually directed toward the same sex. A bird points bill upward when in close proximity of another bird, sometimes performed during a full run toward the recipient.

Head-down: One bird approaches another and assumes a head-down posture, raising the feathers on the nape of its neck; similar to the display of the Brown-headed Cowbird.

Courtship

Bow display: Directed towards female by male. He first lifts the feathers on the back of the neck, followed by those of the upperparts, and finally those of the underparts. The tail is brought forward and under the body while the wings are arched slightly and head is bent.

Hover display: The aerial version of the Bow display. Male flutters over female up to a meter high, returning to the ground to perform Bow display, or to repeat Hover display.

Nesting

Lays eggs in nests of other birds, eighty-two host species have been confirmed. Thirty-two of these species have actually reared Bronzed Cowbird young. Typical host birds nest in brushy habitats in semi-open to open ranch and farm areas as well as in residential areas. More than one female may leave eggs in a host’s nest; laying females frequently pierce eggs already in the host’s nest.

Height: Dependent upon the host species. Bronzed Cowbirds typically select birds that nest in brush, particularly orioles, tanagers, flycatchers, buntings, grosbeaks, and, more rarely, thrashers and thrushes.

Eggs: 1–3 unmarked light-bluish to greenish eggs are laid in a nest, usually early in the morning.

Incubation Period: 10–12 days.

Nestling Period: 11 days.