BIRDS OF THE ST. CROIX RIVER VALLEY 



9 



Fig, 3. Major vegetative communities of the St. Croix River Valley, adapted from 

 Curtis (1959) and Marschner (1930). 



tural 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 pre- 

 cipitation, temperature, and geology, this zone of vegetational range ex- 

 tremes 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 (19746) demonstrated the influence of the 

 Tension Zone on the breeding range limits of the alder and willow fly- 

 catchers. 



Nineteen distinct habitat categories have been identified. Each is pre- 

 sented here in terms of size, distribution, floral characteristics, and charac- 

 teristic 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. 



