Be 6 
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404 
winter in northern Illinois than in central, and more so in central than in 
southern, but evidently the larger the number of summer species re- 
tiring southward in winter the larger will be the number that will leave 
a few representatives in especially favorable haunts. The summer resi- 
dents which seemingly linger into winter are very likely migrants from 
the northern edge of the summer area of the species, accustomed to a 
lower temperature throughout the year than their fellows. It seems 
possible, if not probable, that the whole mass of a species moves in 
migration, not as a mob, but as a more or less fixed array, those northern- 
most at the beginning of the movement being also northernmost at its 
end, each part of the array seeking the kind of climate to which it has 
become accustomed. If this is the case, the summer birds of northern 
Illinois whose area of summer residence extends farthest northward 
will be most likely to leave a recognizable number of representatives in 
northern Illinois in winter, and this will be especially likely if the species 
contains a large number of birds. Other things being equal, members 
of the migrating species represented by the largest numbers will be most 
likely to be found in winter farthest north. 
Our tables of residence classification for the three sections of the 
state and the four seasons of the year show the effect of change of season 
on the geographical distribution of the species tabulated, and the first 
division of each table is thus the essential part of it; in this the species 
is the unit and not the bud. The number of birds belonging to a 
species is a secondary matter in determining its geographical distribu- 
tion, and as the second and third sections of our tables deal with such 
numbers, they have relatively little significance in a residence classifica- 
tion. 
WINTER Brrps IN Hasitats 
The acreages of some of the vegetation areas covered by the winter 
operations of our bird survey—namely, swamps, 22 acres, orchards, 38 
acres, yards and gardens, 26 acres—are so small as to be of doubtful 
value for our purposes, but those of cereal and forage crops, woods, 
shrubbery, and waste and fallow lands, ranging from 250 to 1790 
acres each, may be used to fair advantage. There is, of course, 
much less to differentiate them as bird resorts when vegetation is dead 
over all of them and the ground is often covered with-snow than there 
is in the varied and fruitful summer season. The gregarious habit of 
most winter birds calls for a larger number of observations as essential 
to dependable averages ;.the character of the season has much to do with 
the abundance and southward range of several of our typical winter 
residents ; and it is only by rare chance that the data of a single winter 
can approximate an average for the winters of a decade. Nevertheless 
our recorded numbers from different habitats seem worthy of report 
as a means of depicting the contrast in the bird population of the state 
