The historic range of the bison (Bison bison) on the Great Plains has been well documented. In Colorado the range of bison included both the eastern grasslands and higher-elevation ridges and mountain parks, up to an elevation of 3900 m. Based on the commensal relationship of the brood-parasitic Brown-headed Cowbird (Molothrus ater) with the bison, we suggest that the cowbird had a larger historical elevational range in Colorado than previously known and consequently has had a long-term host-parasite relationship with high-elevation breeding songbirds.
Chace, Jameson F. and Cruz, Alexander
"Range of the Brown-headed Cowbird in Colorado: past and present,"
Great Basin Naturalist: Vol. 58
, Article 4.
Available at: http://scholarsarchive.byu.edu/gbn/vol58/iss3/4