375 



together, 550 to the square mile, and the 64 less abundant species, 

 taken together, numbered 95 birds to the square mile, or i to every 

 6}i acres. The latter species are evidently negligible as general 

 factors in the ecological system, and attention need be given, in 

 discussing the birds of the state as a whole, only to the 21 specie? 

 common enough to produce some appreciable general effect. Given 

 in the order of their abundance they are as follows. 



Variation with Latitude 



The English sparrow decreases in abundance from north to 

 south, from 147 to the square mile in northern to 113 in central, 

 and 82 in southern, Illinois. One hundred sparrows in the northern 

 part of the state are thus represented by yy in the central and 56 

 in the southern part.* The native summer residents, on the other 

 hand, increase in numbers from north to south, the birds per square 

 mile being 464, 537, and 600 for northern, central, and southern Illi- 

 nois, respectively. That is, 100 native birds in northern Illinois were 

 represented in midsummer by i [6 in central and 129 in southern 

 Illinois. The decrease in English sparrows from north to south is 



*Since the above was written, my attention has been called, by Dr. Hans 

 Gadow, to the fact that in Europe also this sparrow diminishes in number south- 

 ward. 



