306 



species, and equally little knowledge, in consequence, of the total 

 significance of birds as a class. We do know fairly well (owing, in 

 part, to the early work of this Laboratory*, but mainly to that of the 

 United States Biological Survey) the principal features of the food 

 of many species of our common birds, but we can not lay these data 

 together for an intelligent estimate of the total effect of the life of 

 birds on their environment except on the supposition that the various 

 species are about equally abundant wherever they occur. That this 

 is not the fact is obvious to every one, and it must be equally obvious, 

 consequently, that until we know how abundant, on an average, the 

 various species are in the various parts of the country and throughout 

 the country at large, we can make little definite application, either 

 scientific or strictly practical, of the knowledge we now have. Our 

 present information in this field is like a chain, one of the links of 

 which is missing and has been replaced by a piece of twine. To 

 substitute iron for cotton at this point is the object of the studies now 

 in progress in Illinois on the local distribution, average numbers, and 

 ecological preferences of the various species of Illinois birds. 



The Field Method. 



To this end, after a preliminary quantitative study made in 

 1905-06 of the bird life of a single limited tract — a 400-acre stock 

 and grain farm in central Illinois — a systematic program of field ob- 

 servation and statistical record was entered upon last August, with 

 complete arrangements for its continuance through one entire year. 

 Two acute and thoroughlv reliable ornithological observers — one of 

 whom, Mr. A. O. Gross, although still an undergraduate student in 

 the University of Illinois, has had several years' experience as a col- 

 lector and observer of birds — were sent into the field under 

 instructions to traverse the state in various directions, traveling al- 

 ways in straight lines and always thirty yards apart, and noting and 

 recording the species, numbers, and exact situation of all birds flushed 

 by them on a strip fifty yards in width, including also those crossing 

 this strip within one hundred yards to their front. No attention is 

 paid by them, for this purpose, to any other birds. 



As they are able to recognize with accuracy all species of Illinois 

 birds at sight, and most of them by song, their movement is like that 

 of a gigantic sweep-net 150 feet wide and 300 feet deep, so drawn 

 across the country day by day as to capture every bird which comes 

 in its way — with this difference, that the birds are not actually caught 



*See Bull. 111. State Lab. Nat. Hist.. Nos. 3 and 6, Vol. I. 



