144 
The Food of Birds. 
the contents of the stomachs of one hundred and forty- 
nine birds,* upon which table certain differences of food 
are clearly shown. Now, if these differences were local 
and accidental, they would undoubtedly tend to disappear 
when larger numbers of specimens were examined; but 
if they are specific and constant, they should be made the 
more evident, on the whole, the larger the number of 
specimens taken. The table on page 147 presents data 
derived from three hundred and fifteen specimens, cover- 
ing considerably more time and area than the table in the 
Transactions. If the difference between the food records 
of the various species are now greater than before, we 
may conclude that the differences noted are real and not 
artificial. If they are less, on the other hand, the whole 
question is still unsettled. The differences apparent in 
the later table may be specific, but there is no proof of it. 
In order to apply this crucial test as fully as possible, I 
have selected twelve food elements in which the differ- 
ences were most apparent, and, taking the species in 
pairs, have ascertained the sum of the differences of the 
ratios of these elements for each pair separately, first 
from the old table and then from the new. In every case 
but one the sum of these differences has been much larger 
by the new table than by the old, thus proving conclusive- 
ly that the species appear to diverge in food habits the 
more widely the greater the number of specimens studied. 
For example, the differences of the selected elements as 
shown in the original table of seventy-eight robins and 
catbirds, amounted to sixty-four per cent.; and by the 
new table of one hundred and eighty-four birds, to 
eighty-two per cent. A similar comparison of the food 
of the catbird and hermit thrush gives one hundred and 
twenty-five as the sum of the differences of the old table 
of fifty-five birds, and one hundred and fifty-five as the 
sum of the differences of the new table of ninety-one 
birds. Taking the catbird and the brown thrush, we have 
sixty-four and ninety-nine parts for the old and new ta- 
bles respectively, the first for sixty-five birds and the 
second for one hundred and thirty-four; while the brown 
thrush and wood thrush give seventy-eight and eighty- 
* Trans. 111. Hort. Society. 1879, N. S. r Vol. 13, p. 163. 
