Birders

The historical range of the cowbird prior to European settlement was restricted to the open short-grass prairies of the Midwest. Presumably as a result of large-scale deforestation, introduction of cattle, and other activities associated with European settlement, the cowbird's range expanded rapidly eastward and the species was widespread throughout the eastern U.S. by the late 1800's.

A similar expansion happened in the west during the 1900's resulting in their distribution being spread throughout southern Canada and the lower 48 states. Cowbird abundance has continued to increase dramatically throughout the breeding range since 1900, possibly due to greater overwinter survival from an increase in food supply (residual agricultural grain) and increases in reproductive success from exposure to new, vulnerable host species.