We describe sexual dichromatism in the plumage of juvenile Brown-headed Cowbirds (Molothrus ater obscurus) collected in Ventura County, California in 1991. The dichromatism was based on two colorimetric features. The first was the contrast between the greater underwing coverts and the remiges, with males having darker coverts, and therefore greater contrast between their underwing coverts and remiges. The second was the color and extent of pale feather edges on the underside of the distal portion of the carpometacarpal joint, with males having wider, slightly darker feather edging. Five observers were used to test the reliability of these characters in a sample of museum wings from cowbirds of known sex. They were 88–94% accurate at determining the sex of 33 juvenile cowbirds (17 males, 16 females) using these two colorimetric characters. This technique was slightly more reliable than wing chord (84.8% accurate) and tarsometatarsus (also 84.8% accurate) measurements in differentiating the sex of our sample individuals.