No. 500] MIDSUMMER BIRD LIFE OF ILLINOIS 509 



ber to March 15, birds averaged 384 to the square mile in 

 northern Illinois ; from December 23 to March 21, 582 to 

 the mile in central Illinois; and from February 6 to 

 February 21, 832 to the mile in southern Illinois, numbers 

 related to each other as 100, 151 and 217. Indeed, we 

 find birds more abundant in extreme southern Illinois in 

 the mid-winter period of 1906-07 than in the mid-summer 

 period of 1907, averaging at the rate of 122 birds in the 

 former season to each hundred in the latter. 



If we take into account the numbers for the whole year, 

 there are, for every hundred birds in the northern part 

 of the state, 133 for central and 181 for southern Illinois. 



100 116 129 



Ull< All-hinls 



The bobolink was a distinctively northern bird, occur- 

 ring in the ratio of 24 to the square mile in northern 

 Illinois, and not at all in either of the other sections. The 

 mocking-bird, on the other hand, was almost exclusively 

 southern, being represented by 8 birds to the square mile 

 in the southern section, by only 1 specimen seen in cen- 

 tral Illinois, and not at all in the northern part of the 

 state. 



Migration Waves 

 In a paper published last April under the title "An 

 Ornithological Cross-section of Illinois in Autumn," 4 I 

 gave the data and results of a trip across central Illinois 

 made by Gross and Ray during the fall of 1906. A com- 

 parison of the general average of the bird population, 



4 Bull. 111. State Lab. Nat. Hist., Vol. VII, art. 9. 



