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Fig. 3. Major vegetative communities of the St. Croix River Valley, adapted from Curtis (1959) and Marschner (1930).

Man's presence has had a profound impact on the composition, distribution, and extent of these major communities. The Tall Grass Prairie which may have lapped at the shoulders of early settlers is now field and pastures. The prairie is now confined to railroad rights-of-way and small odd corners that would not accommodate a plow. The majestic white pine forest that once blanketed most of the northern regions has been reduced to scattered remnants.

Habitat changes that adversely affect some bird species often encourage expansion of other species. The clearing of forests of northern white pine has allowed a second growth deciduous disclimax habitat type to develop. Land clearing for agriculture was very important to the advance of the greater prairie chicken (scientific names of birds are given in the species accounts) in this region. Early accounts make vivid reference to the large number of greater prairie chickens that occurred after moderate agricultural expansion. However, the rapacious clearing that accompanied intensified agricultural production also caused the demise of the greater prairie chicken as a natural breeding bird.

An important environmental characteristic that affects bird occurrence and distribution is described by Curtis (1959) as the "Tension Zone." Based on a combination of environmental factors including soil type, annual precipitation, temperature, and geology, this zone of vegetational range extremes has a profound effect on the distribution of many bird species in the Valley. Essentially, this zone is the north-south limit area for many boreal forest and oak forest forbs, shrubs and trees. This contact zone was shown by Beimborn (1969) as important to the distribution of at least 14 bird species in Wisconsin. Robbins (1974b) demonstrated the influence of the Tension Zone on the breeding range limits of the alder and willow flycatchers.

Nineteen distinct habitat categories have been identified. Each is presented here in terms of size, distribution, floral characteristics, and characteristic breeding birds. Breeding bird species that apparently reach their greatest density in a specific habitat are marked with an asterisk. Habitat use by migrants will be considered individually with each species account.