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 



