374 



third of this area in suuthern, a fourth in central, and two fifths in 

 northern, Illinois. The field observations began in the south June 

 4, and ended at the north August 23, with the idea of avoiding, so 

 far as possible, by this order of progress, differences due to different 

 seasonal conditions. It \\as not possible, of course, to eliminate these 

 wholly, with only one pair of observers ; and it will tax our ingenu- 

 ity, and sometimes perhaps overtax it, to detect these differences and 

 to distinguish them from those due to mere difference of latitude 

 and of climate corresponding. 



The total surface on which these precise midsummer observa- 

 tions were made was 1/4720 part of the whole state, and the ques- 

 tion at once arises, Was this area sufficient to give these results any 

 general value for the state at large, and, if so, how may we be stire 

 of it? There is, I believe, no mathematical method of determining 

 the sufficiency of these data for generalization purposes, and I know 

 O'f no test at present applicable except that of the general consistency 

 and reasonableness of the totals, averages, and ratios, for the differ- 

 ent districts and seasons, 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 oe 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 i per 

 acre, including the so-called English sparrow\ If we omit the 1,414 

 interloping English sparrows observed — which is a little inore than 

 t8 per cent, of the entire numlier of l)irds — we have remaining 527 

 native birds to the s(|uare mile. The total for Illinois,* on this basis, 

 is 30,750,000 native birds and 5,536,000 English sparrows, or ap- 

 proximately 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 



*A combination of the averages for the three sections of the state, computed 

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

 for differences in area. 



