402 
ComBinrp Most ABUNDANT List, 1906 AND 1907, CenTRAL 
AND NORTHERN ILLINOIS, (83.3 PER CENT. LIST) 
é Numbers Per square 
Species counted mile 
Crow 733 95 
Lapland longspur 675 87 
Prairie horned lark 413 53 
English sparrow 382 49 
Goldfinch 332 43 
Tree sparrow 250 32 
Junco 141 19 
Total 2926 378 
Comparing the foregoing ratios with those of southern Illinois, we 
see in the winter birds of the southern section a difference from the 
northern sections in the number of the more dominant species like that 
to which attention was called in our discussion of the summer birds of 
the state. To make 85 per cent. of the total number of winter birds we 
must take for southern Illinois more than 36 per cent. of the species, and 
for central and northern Illinois only 19 per cent—fifteen species in 
southern Illinois as compared with seven in northern-central. The rela- 
tively greater ecological complexity of the southern part of the state is 
thus reflected, in winter as well as in summer, in the greater number of 
species of birds with numbers large enough to make them important as 
members of the ornithological community. 
The winter birds of central and northern Illinois taken as one area, 
were fewer per square mile by 32 per cent. than those of the summer- 
time (430 to 630), but the southern Illinois birds were 22.6 per cent. 
more numerous per square mile in winter than in summer (836 to 682). 
Our general averages per square mile for the state as a whole were 520 
for the winter and 644 for the summer—the winter Sey: nearly 24 
per cent. smaller than the summer. 
RESIDENCE CLASSES AND THEIR SEASONAL MovEMENTS 
From this point onward, we shall make much use of tables show- 
ing the residence classification of the species dealt with, and a few words 
of general information seem necessary. 
Although the birds of a locality or of an area of moderate size and 
fairly uniform ecological condition, are somewhat definitely divisible into 
the four classes of permanent residents, winter residents, summer resi- 
dents, and migrants (the last term being used for those which pass en- 
tirely through the area in migration), a strict classification on this basis 
is impracticable since the northward and southward range of a species 
. 
