108 The Food of Birds. 



with the same sources of food supply open to them, are 

 found, on the examination of a limited number of stom- 

 achs, to present several characteristic differences of food, 

 so that the investigator can point out definite peculiarities 

 of the food of each species, and finds these peculiarities 

 reasonably constant, year after year, then we may say un- 

 questionably, 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 in- 

 variableness than in those relating to the food, which are 

 nevertheless found to be substantially unvarying, then we 

 may, with considerable force, argue the probability of a 

 like unvarying character in the habits of alimentation. 



"Respecting 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 com- 

 parison with each other, that I have made out certain 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 peculiarities, 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 hes- 

 itate 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 distinguish be- 

 tween them at a glance. In fact, a friend mentions, as 

 I write, two birds whose nests are much more easily dis- 

 tinguished than the birds themselves." 



I have now to add what we may regard as a decisive cru- 



