190 
generalization in this field. Among the minor illustrations of this fact 
is the disagreement of the second year’s data with a conclusion reached 
in an earlier paper* already cited, to the effect that the total number 
of our summer birds increases from north to south in Illinois. This 
seemed to be true by our data of 1907, but it was not at all so by those 
of 1909; and when the averages for both years are brought together, 
as in the preceding table, we see that the numbers per square mile are 
larger for southern Illinois than for northern but are largest of all for 
the central part of the state. 
NuMBERS OF GREGARIOUS AND SOLITARY SPECIES, RESPECTIVELY 
In a search for possible causes of the marked increase of numbers 
throughout the state noted in 1909, we have tabulated separately the 
data for our nine principal gregarious species, namely, the English 
sparrow, quail, mourning dove, crow, bobolink, cowbird, red-winged 
blackbird, crow-blackbird, and goldfinch, and have brought these into 
comparison with the data for the solitary species remaining, with the 
result that the increase in numbers in 1909 is found almost wholly in 
the gregarious group.. While the numbers of the solitary species per 
square mile in 1909 average for the whole summer and the whole state 
6 per cent. larger than in 1907, those for the gregarious species are 
2.39 times as large. 
This fact raises the question whether the indicated increase in num- 
bers was real, or only apparent and due to the insufficiency of our data. 
The unit of observation of the solitary species is the single bird, while 
that of the gregarious species is often a group of companions varying 
from a small flock to many hundreds. A record of a thousand solitary 
birds thus represents a larger number of separate observations than one 
of the same number of gregarious birds, and the averages in the latter 
case are less likely to be valid than in the former. 
The social birds are not always gregarious, however, most of them 
scattering during their nesting season and often when in search of food, 
assembling only at the time of the spring migration and in late summer 
and fall after their young have become independent. That the actual 
number of gregarious birds to a single observation is much smaller than 
might be supposed, is shown by one of our tables which enables us to 
make a comparison between seven gregarious species and seventeen of 
the most abundant solitary species with respect to the average numbers 
recorded for each field or other unit of area covered by the summer 
survey of 190%. By this table it appears that the gregarious birds ranged 
in number per record from 2.4 for the mourning dove to 9.1 for the 
crow-blackbird, with an average of 6 per record for the seven species ; 
while the corresponding numbers for the seventeen solitary species 
ranged from 1.3 for the brown thrasher and the red-headed woodpecker 
*The Midsummer Bird Life of Illinois, loc. cit., p, 375-376. 
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