The Food of Birds 143 
into which these substances are classified, and the part 
they take in supplying birds with food. 
As with all animals, certain mineral salts are very 
necessary to a bird’s existence, such as the substances 
from which the calcium phosphate for the bones, and 
the calcium carbonate for the shells of the eggs, are de- 
Fig. 108.—Finch, a bird with heavy, thick bill adapted for crushing seed. 
rived. The gravel and pebbles swallowed by birds in 
the course of their daily feeding should hardly be men- 
tioned in this connection, as this is only done for the 
mechanical assistance, derived from the hard surfaces, 
in triturating the food. 
Vegetable-feeders form a large group among birds, and 
they alone would offer an interesting field for study, as 
