Montana Field Guide

Brown-headed Cowbird - Molothrus ater

Length: 19-cm; heavy conical bill and a brown iris; walks with the squared tail uptilted. Adult male has a plain brown head, neck, and upper chest, with the rest of the plumage glossy greenish black. Adult female is gray-brown above, paler below. Juvenile is paler above, heavily streaked below, with pale edgings on the feathers on the back; young males molting to adult plumage are patchy black, brown, and buff; mass of adult male is 39-57 g, female is smaller (NGS 1983). EGGS: vary from pure white to bluish-white; entire egg is covered with chocolate brown or yellowish-tan specks or blotches, which are often heaviest around the larger end (Friedmann 1929). Eggs average 21.5 by 16.4 mm (Bent 1958), with a mass of 3.2-3.4 g (Walkinshaw 1983).

Montana Range

In the Bozeman area, normal migration periods are May 8 to 30 with no discernible movement in the fall.

Species prefers areas with low or scattered trees among grassland vegetation—woodland edges, brushy thickets, prairies, fields, pastures, orchards, or even residential areas.

Diet consists mainly of anthropods and seeds.

Species is a brood parasite; nests are chosen by females, but are that of another species. Care given to cowbird eggs and young is provided by the host and reflects characteristics of that species. Brood consists of four to five eggs; incubation time 10-13 days. Young able to fly 10-11 days after hatch.