N'o. 500] MIDSUMMER BIRD LIFE OF ILLINOIS 511 



the excess of 42 birds per square mile of fall over summer. 

 This temporary increase of 8 per cent, in autumn in the 

 average number of our birds is thus evidence of a wave 

 of condensation running southward in consequence of the 

 earlier beginning and more rapid development at the 

 north of the annual fall migration. 



This contrast of the number of the resident summer 

 population with that of the fall migration period is still 

 more clearly and strongly shown by a comparison of the 

 totals of all our central Illinois observations in mid-sum- 

 mer and in fall, respectively. These average 1.07 birds to 

 the acre for the period from July 9 to September 21, and 

 2.31 per acre for the interval between the 1st and the 

 26th of October. That is, more than twice as many birds 

 per acre were seen in October of this year as in July, 

 August and September. 



The data of the spring migration of 1907 are unsatis- 

 factory owing to the extraordinary character of the sea- 

 son, and the consequent repeated interruption and remark- 

 able prolongation of the movement. Nevertheless, they 

 indicate a larger population during the early part, at 

 least, of this migration period also than either before or 

 after it. A trip down the eastern side of the state from 

 Cook to White county, begun March 26 and ending April 

 11, gave an average of 1.34 birds to the acre— a number to 

 be' compared with our mid-summer average for the whole 

 state, which is 1.03. That is, the average early spring 

 population of this exceptional year was 30 per cent- 

 greater than the average of the summer following. On 

 the other hand, a trip across central Illinois between 

 April 20 and May 29, still within the migration period, 

 gave us, for 5 1/3 square miles of area, an average of only 

 .89 per acre— less than even the mid-winter average of 

 .91 for the same part of the state. 



Vegetation of the Inspection Area 



As a basis for a more precise account of the distribu- 

 tion of birds as a whole and of the more important 



