No. 500] MIDSUMMER BIRD LIFE OF ILLINOIS 



507 



averages and ratios, for the different districts and sea- 

 sons, the presence or absence of which each can readily 

 see for himself as this discussion proceeds. If the data 

 of observation are insufficient for the uses made of them, 

 there will be a random variability and inexplicable 

 irregularity in my statistical summaries which we shall 

 not fail to notice. 



General Product of the Survey 

 Gross and Ray identified during the summer, on the 

 territory covered by their data, 7,740 birds, belonging to 

 85 species. This is at the rate of 645 birds per square 

 mile, or almost precisely 1 per acre, including the so- 

 called English sparrow. If we omit the 1,414 interloping 

 English sparrows observed— which is a little more than 

 18 per cent, of the entire number of birds— we have 

 remaining 527 native birds to the square mile. The total 

 for Illinois, 2 on this basis, is 30,750,000 native birds and 

 5,536,000 English sparrows, or approximately 14 summer 

 resident birds to each person in this state living in the 

 country or in towns of less than 25,000 inhabitants. 



Of the 85 species represented by the 7,740 birds 

 recognized on these trips, the 21 most abundant species 

 were represented by 6,596 birds. That is to say, 85 per 

 cent, of the birds belonged to 25 per cent, of the species. 

 The 21 more abundant species numbered, taken together, 

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

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

 1 to every 6f 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 species common enough 

 to produce some appreciable general effect. Given in the 

 order of their abundance they are as follows : 



puted separately, the data for the sections being differently weighted to 

 compensate for differences in area. 



