308 



yond Danville, 111., to Quiney, on the Mississippi River. It has 

 to do with autumnal conditions in the central part of the state, 

 and is merely preliminary to a comprehensive report on the 

 vphole investigation. 



The entire distance covered by these observations is 191.86 

 miles, and the strip from which all birds were accurately de- 

 termined and numbered was 150 feet in width for this whole 

 distance. The area thus covered was 3519 acres, or 5i square 

 miles. It included every kind of surface, soil, and vegetation 

 traversed by the observers, with the exception of forests of too 

 lofty or too dense a growth for a complete and certain recogni- 

 tion of their bird population. 



The whole number of birds identified was 4804, of which 

 1620 were English sparrows and 3184 were of native species. 

 The average number of bii'ds seen was 25 for each mile of the 

 trip, which is 1.36 for each acre covered, or 874 for each square 

 mile. The English sparrows averaged .46, and the native spe- 

 cies .9, per acre, or 295 per square mile for the sparrows and 579 

 per square mile for the native birds. The total number of 

 species recognized was 93; but 90 per cent, of the individual 

 birds seen, belonged to 20 of these species, leaving but 10 per 

 cent, for the other 78 species. Indeed, 15 species included 85 

 per cent, of the individual birds observed, leaving for the other 

 81 species but 728 birds — an average of 130 liirds per square 

 mile, or one bird to each five acres. 



It is evident, consequently, that the real dynamic signifi- 

 cance of the birds of this district at this time was to be found 

 wholly in the fifteen most abundant species, the remainder be- 

 ing virtually negligible as a general ecological factor.* These 

 fifteen species are arranged in the order of their frequency in 

 the following table, which shows for each the number of indi- 



* A species repreeented by a relatively small number of birds may have a special 

 ecological significance if it is concentrated in a special class of situations; and may, 

 indeed, be especially important ecologically if the class of situations in which it is con- 

 centrated is especially important. This aspect of the general problem must be reserved 

 for discussion when a larger mass and a more comprehensive variety of data are available. 



