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THE AMERICAN NATURALIST . [Vol. XLII 



tion of the birds of Illinois during the relatively stable 

 period of their summer residence— the time between the 

 conclusion of the spring migration and the beginning of 

 the fall movement to the southward. It is a period of 

 breeding and steady habitation for our most permanent 

 and characteristic bird population, and will best help us 

 to an understanding of the main normal ecological signif- 

 icance of Illinois birds. 



The Akea of Observation 



The total distance traveled by my observers on these 

 various mid-summer trips was 428 miles (omitting frac- 

 tions), of which 141 miles was in southern Illinois, 112 in 

 central, and 175 in northern. The total area covered by 

 this strict census of the bird population was a trifle over 

 12 square miles, or 7,693.5 acres— 33 per cent, of this 

 acreage being in the southern, 26 per cent, in the central, 

 and 41 per cent, in the northern, part of the state— or 

 approximately a third of this area in southern, 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 was not possible, of 

 course, to eliminate these wholly, with only one pair of 

 observers; and it will tax our ingenuity, and sometimes 

 perhaps overtax it, to detect these differences and to dis- 

 tinguish them from those due to mere difference of lati- 

 tude and of climate corresponding. 



The total surface on which these precise mid-summer 

 observations were made was 1/4,720 part of the whole 

 state, and the question at once arises, Was this area suf- 

 ficient to give these results any general value for the state 

 at large, and, if so, how may we be sure of it? There is, 

 I believe, no mathematical method of determining the 

 sufficiency of these data for generalization purposes, and 

 I know of no test at present applicable except that of the 

 general consistency and reasonableness of the totals, 



