378 



and more rapid development at the north of the annual fall migra- 

 tion. 



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 ob- 

 servations in midsummer 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 ist and the 26th of 

 October. That is, more than twice as many birds per acre were 

 seen in October, 1907, as in July, August, and September. 



The data of the spring migration of 1907 are unsatisfactory ow- 

 ing to the extraordinary character of the season, and the consequent 

 repeated interruption and remarkable 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 midsummer aver- 

 age 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 midwinter average 

 of .91 for the same part of the state. 



Vegetation oe the Inspection Area 



As a basis for a more precise account of the distribution of birds 

 as a whole and of the more important species, it will be necessary 

 to consider the vegetable covering of the soil, since there is little 

 else in Illinois by which different portions oi its area may be dis- 

 tinguished. The territory traversed by my observers, it need hardly 

 be said, was almost wholly under cultivation. Excluding only for- 

 ests in which the trees were too high, or the undergrowth was too 

 dense, to permit a full and accurate census of the birds, the terri- 

 tory reported upon was chosen wholly at random, and the total 

 for each division of the state seems sufficient to give us, with the 

 exception just mentioned, a fair sample of its crops and surface con- 

 ditions. The areas from which all the birds were determined were 

 3,172 acres for northern Illinois, 2,117 acres for central, and 2,504 

 acres for southern. 



