1 7 o Wild Birds 



On the fourth day the incubating instinct was then fully de- 

 veloped, and lasted until two of the eggs were hatched, on about 

 the tenth day of July. The period of incubation was approxi- 

 mately twelve days, and when three days old the eyes of the 

 little birds had begun to open, and the feather-tubes of their 

 wings had appeared. In still another nest of this Sparrow, in 

 which two eggs only were laid, incubation did not begin until the 

 third day. 



For ages the greatest secrets of the living world have been 

 securely locked in the egg, and it is only in recent times that sci- 

 ence, with any degree of success, has succeeded in partially open- 

 ing them to view. Development begins in the warm body of the 

 parent bird, and in most cases comes to a stand the moment 

 the egg is laid. The physiological zero-point of temperature at 

 which no further development takes place has been accurately 

 determined in the domestic fowl. 1 When heat is again applied 

 by the brooding hen or the artificial incubator (the normal 

 temperature for development of the fowl's egg being about 

 103 F.) the engines of cell -life are again started, and the won- 

 derful process of orderly growth, with the ensuing changes, is 

 taken up anew. If the fires are again banked and the engines 

 stopped by withdrawal of the heat for any length of time, it is 

 impossible to start them again, so we kill the egg, and perhaps 

 the goose which might have laid another, at the same time. 

 The strong brooding instinct in birds, which often supplants the 

 usually strong instinct of fear, is thus adapted to meet a very 

 urgent need. 



The yolk, and later the white or albumen, furnishes a natural 

 store of food and energy, which, like money in the bank, can be 

 drawn upon by the growing cells or embryo, until, enclosed in 

 the stomach of the little bird, it is gradually exhausted, when 

 the latter is ready to receive food from another source. 



Owing to some perversity of instinct, the domestic fowl will 



occasionally eat its own eggs; wild birds often desert theirs 



through fear, or, as more commonly happens with sea fowl, lay 



them in places of insecurity, but as a rule these fragile objects 



1 According to Professor C. L. Edwards, this lies between 68 and 70 F. 



