The sexing by plumage of 4316 first year cowbirds [Molothrus ater] showed a 1:1 sex ratio. An additional 2695 birds could not be sexed by plumage. First-year birds sexed by plumage revealed that 99% of the females had wing chords of 99 mm or less and 98% of the males had wing cords of 103 mm or greater. Using these criteria approximately 90% of all birds could be sexed by wing length alone.