Numerous studies have described nest parasitism by Brown-headed Cowbirds (Molothrus ater) as it relates to a single host species or to an assemblage of hosts in a given region. Most of the regional studies have encompasses short spans of time, seldom more than 3 or 4 years. A few outstanding accounts, such as Friedmann’s works and Mayfield’s paper on invasion patterns, provide historical information about the impact of cowbird parasitism on host species. Our study is unique in that it provides data on the frequency of cowbird parasitism for a single region over a span of 68 years. In this paper, we have used data collected at the University of Michigan Biological Station between 1911 and 1978 to describe the relationship between the parasitic Brown-headed Cowbird and 57 species of passerines that represent actual or potential hosts in northern Michigan. All of the species considered are open-nesters, as opposed to cavity-nesters. Species using preformed or excavated cavities as nest sites were excluded because information was seldom available on nest contents.