16 OUR DOMESTIC BIRDS 



Domestic birds which produce many eggs require special sup- 

 plies of food containing lime to make the shells. Until within 

 a few years it was universally believed and it is still commonly 

 supposed that birds needed grit to take the place of the teeth 

 nature did not give them, and assist in the grinding of the food 

 in the gizzard. Many close observers now reject this idea 

 because they find that birds supplied with digestible mineral 

 foods do not eat those that are not digestible. A bird does 

 not need teeth to grind its food, because it is softened in 

 the crop and the gastric juice acts upon it before the grinding 

 process begins. 



Peculiarities of birds' eggs. The only animal foodstuff pro- 

 duced in a natural package, easily preserved and handled, is the 

 egg. In the vegetable world we have a great many such things 

 fruits, seeds, roots, nuts, with coverings of various textures to 

 protect the contents from the air. In all of these the material 

 stored up is either for the nourishment of the seeds in the first 

 stages of growth as plants, or for the nourishment of a new or 

 special growth. An egg is the seed of an animal. All animals 

 produce eggs, but in mammals the new life originating from 

 the egg goes through the embryonic stages within the body of 

 the parent, while in insects, fishes, reptiles, and birds the egg 

 is laid by the creature producing it before the embryo begins 

 to develop. 



In mammals the embryo grows as a part of the body of the 

 parent, the substances which build it up coming from the parent 

 form as they are needed. In birds a tiny germ the true egg 

 is put, with all the material needed for its development as an 

 embryo, in a sealed package, which may be taken thousands of 

 miles away from the parent, and, after lying dormant for weeks, 

 may begin to grow as soon as the proper conditions of temper- 

 ature are applied. The food value of the germ of an egg is in- 

 appreciable. We use the egg to get the material stored up in 

 it for the young bird which would come from the germ. 



