108 
The Food of Birds. 
side, with the same sources of food supply open to them, 
are found, on the examination of a limited number of 
stomachs, to present several characteristic differences of 
food, so that the investigator can point out definite pecu- 
liarities of the food of each species, and finds these pecu- 
liarities reasonably constant, year after year, then we 
may say unquestionably, without going farther, that 
there is a fixity of food-habits in this group of birds 
which will allow us to reason from the data observed. 
“Second, if there are any other habits of the species 
in which there does not seem to be any greater reason for 
invariableness than in those relating to the food, which 
are nevertheless found to be substantially unvarying, 
then we may, with considerable force, argue the proba- 
bility of a like unvarying character in the habits of ali- 
mentation. 
“Bespecting the first of these tests, you will see, when 
I sum up the food of the family now under consideration 
and bring the data respecting the various species into 
comparison with each other, that I have made out cer- 
tain very well-marked specific differences of food, even 
among those eating at the same table ; that the different 
species of this group, while agreeing in many particulars 
of food as they do in structure, present also certain pecu- 
liarities, so marked that I can usually determine the 
species by the contents of three or four stomachs. 
“For the second test we may properly use the nesting 
habit. There seems to be no more cogent reason why one 
species should select from the same storehouse different 
materials for its nest from those used by another closely 
allied species of nearly the same size and similar general 
habits, and building in the same locality, than why each 
should use a similar fixed discrimination in selecting its 
food. Yet no expert, scarcely a schoolboy even, will hesi- 
tate a moment between the nest of a robin and that of a 
catbird; and the descriptions of the two given in the 
books are so different as to enable any novice to distin- 
guish between them at a glance. In fact, a friend men- 
tions, as I write, two birds whose nests are much more 
easily distinguished than the birds themselves. ” 
