TOPOGRAPHY AND DISTRIBUTION 



S9 



represented. Two factors operate to bring the number of species 

 and subspecies to its present total : First, the great changes of tem- 

 perature, which in summer permit southern species to reach our 

 borders, and in winter bring down the boreal species from the 

 north, and second, the intergradations of eastern and western, or 

 even southern forms, which, as in the Red-tailed Hawk, contribute 

 besides the species (Buteo borealis borealis) three subspecies (kri- 

 derii, calurus, and harlanii). 



With the exception of Winnebago and Worth counties, parts of 

 Osceola, Dickinson, Emmet, Kossuth, Hancock, Cerro Gordo, Floyd, 

 Mitchell, Howard, Winneshiek and Allamakee, which are ascribed 

 to the transitional zone, Iowa lies entirely within the Upper Austral 

 zone (4th Provisional Map of the Biological Survey, 1910). The 

 mean temperature approximates 47.5 degrees and the difference in 

 mean temperature between northern and southern Iowa is less than 

 five degrees. The difference between extreme temperature in sum- 

 mer and winter is great, not unusually showing a maximum of 

 nearly -|-100 degrees in summer and a minimum of 25 degrees 

 in winter. 



FIG. 13. Life zones as given by Merriam. (4th Provisional Map, Biological Survey, 

 1910.) Shaded area represents Transitional zone, white area represents Upper 

 Austral zone. 



Since the identification of Hawks and Owls is rendered difficult 

 by the usual shyness of these birds, the confusing common names 

 given them (such as "Chicken Hawk," which may mean one of three 



