During a 2-year study of brood parasitism by the Brown-headed Cowbird (Molothrus ater) in 1977 and 1978 in Colorado, I noted substantial differences in the cowbirds' range from that described in the earlier literature.
Although the Brown-headed Cowbird is among the bird species described by the first naturalists in Colorado (Bailey and Niedrach 1965, Marsh 1968), virtually all observations of cowbirds at high altitudes are recent. Bailey and Niedrach (1965) described the Brown-headed Cowbird as a resident of the foothills, ranging up to 2134 m. Davis (1969) described cowbirds as common at lower elevations in western Colorado. Only Cooke (1897) mentioned cowbirds breeding up to 2440 m, but he considered them to be confined primarily to lower elevations. Warren (1980) was cited by Sclater (1912) as having observed cowbirds in Routt and Grand counties. However, Warren’s expedition was for the purpose of mammal collecting and no locations of bird sightings were published. Sclater (1912) also considered the Brown-headed Cowbird to be a bird of the plains and foothills.