376 



not sufficient to offset the increase in the native species, the total 

 numbers per square mile for all summer birds in the three sections 

 of the state being 6io, 650, and 682 — or 100 birds in northern for 

 107 in central and 112 in southern Illinois. 



This same gradation was much more pronounced in the record 

 of the winter residents. From the last of November 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 midwinter 

 period of 1906-07 than in the midsummer period of 1907, averag- 

 ingf 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. 



Birds by Sections 



The bobolink was a distinctively northern bird, occurring 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 i specimen seen in cen- 

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



AIiGRATioN Waves 



In a paper published in April, 1907, under the title "An Ornitho- 

 logical Cross-section of Illinois in Autumn,"* I gave the data and re- 



*Bull. 111. Stale Lab. Nal. IHst., Vol. VII, Art. 9. 



