198 
29 per cent. for southern Illinois. When, however, we bring together 
our data for the two seasons, as in the table on page 197, and compare 
sectional numbers for the different species, we find that ten species 
sufficiently abundant to make them available for comparison show a 
marked progressive increase in numbers from north to south, and that 
eleven species show a decrease in that direction as follows: 
THE MORE ABUNDANT BIRDS WHICH SHOW A 
PROGRESSIVE CHANGE IN NUMBERS IN SUMMER 
FROM NorTH TO SOUTH IN ILLINOIS 
(DATA OF 1907 AND 1909) 
Increase Decrease 
Quail English sparrow 
Mourning dove Red-headed woodpecker 
Meadowlark Flicker 
Orchard oriole Prairie horned lark 
Lark sparrow Crow 
Field sparrow Bobolink 
Dickcissel Bronzed grackle 
Maryland yellow-throat Vesper sparrow 
Mockingbird Grasshopper sparrow 
Brown thrasher Barn swallow 
Robin 
The general summer range of two of the above species—the mocking- 
bird and the bobolink—is so definitely limited as to make their pre- 
ponderance in southern and northern Illinois respectively a matter of 
course. Indeed, it seems probable that in so level and uniform a state 
as Illinois, where there are few topographical barriers to the distribu- 
tion of birds, the contrasts in numbers between the north and the south 
are mainly due to differences in the general range of the species, those 
whose centers of general distribution lie to the northward of the state 
diminishing in number in Illinois toward the southern boundary of 
their midsummer range, and the Illinois numbers of those whose dis- 
tribution centers lie southward increasing in that direction. 
The degree of overlapping and intermingling of northern and 
southern species in central Illinois will vary somewhat with the character 
of the season, as the different species differ in susceptibility to peculiari- 
ties of temperature, humidity, and other features of the spring and 
summer weather. Variations in the abundance or scarcity of the prin- 
cipal elements of the food of the various species may have a similar 
effect to limit or extend their midsummer distribution in a way to affect 
them variously. Those, for example, with a fixed and narrow range of 
food preferences will be severely checked in their migrations by a local — 
deficiency of their favorite food, while others, with less discriminating — 
tastes, may find the deficiency of certain food elements compensated 
by an unusual abundance of other food. 
