Article: VT. — Tlic Midsummer Bird Life of Illinois: A Statis- 

 lieal Stiidy.^ By Stephen A. Forbes. 



In the course of a statistical survey of the bird population of the 

 State of Illinois, begun with a view to a better knowledge of the 

 significance of birds in the economy of nature, two field observers, 

 A. O. Gross and H. A. Ray, engaged in this work as assistants on 

 the State Natural History Survey, spent virtually a month of the 

 summer period of 1907 in each of the three principal sections of 

 the state — June in southern, July in central, and August in northern, 

 Illinois. Selecting in each section a locality typical for that part of 

 the state, they made regular trips on foot in various directions and 

 to various distances, traveling always thirty yards apart, and noting 

 as they went the species and numbers of all birds flushed by them 

 on a strip fifty yards in width, including likewise those flying across 

 this strip within a hundred yards to their front. They kept record, 

 also, by means of mechanical counters, of the distances traveled over 

 each distinguishable kind of area, commonly marked by the crop 

 which is borne. 



The present paper is a report of a few of the more general re- 

 sults of a study of the materials thus brought together, illustrating 

 the numbers and ecological distribution of the birds of Illinois dur- 

 ing 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 significance of Illinois birds. 



The Area of Observation 



The total distance traveled by my observers on these various 

 midsummer trips was 428 miles (omitting fractions), 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 



^Reprinted from the American Naturalist, Vol. XLII, August, i( 



