A few Eurasian Tree Sparrows were brought to St. Louis, Missouri, in the nineteenth century as part of a shipment of European songbirds imported from Germany. The birds were destined for release as part of a project to enhance the native North American avifauna. Set free in late April 1870, they bred successfully and gradually established a permanent presence in the Americas. Although the Eurasian Tree Sparrow is widely distributed throughout much of Europe and Asia, it has enjoyed only modest success in colonizing its new homeland, and its range has remained localized to extreme eastern Missouri, west-central Illinois, and southeastern Iowa. Typically a commensal of humans, it has, in part, been displaced from urban centers by another introduced species, the larger, more pugnacious House Sparrow (Passer domesticus). Today, its preferred habitat includes wooded urban parkland, farms, and rural wood lots.
The Eurasian Tree Sparrow, as with the House Sparrow, has been studied extensively in its native range, and Summers-Smith ( Summers-Smith 1995 ) provides an excellent introduction to the voluminous literature of the former. In North America, research has largely concerned breeding success ( Anderson 1978b ) and emerging differences between ancestral German birds and the New World population resulting from the constraints of a small founder population. Included are studies of changes in morphology (Barlow Barlow 1973 , Barlow 1980a ; St. Louis and Barlow 1991 ); genetic make-up (St. Louis and Barlow St. Louis and Barlow 1987 , St. Louis and Barlow 1988 ); and song ( Lang 1995 ; Lang and Barlow Lang and Barlow 1987 , Lang and Barlow 1997 ). This account emphasizes North American data, where available.