NATURAL HISTORY. 299 



" Some to the mdc protection of the thorn 

 Commit their feeble offspring : the cleft tree 

 Offers its kind concealment to a few, 

 Their food its insects, and its moss their nests." THOMSOX. 



960. Why do the number of eggs laid by the different 

 species of birds vary so considerably? 



The number of eggs laid by birds is wisely and beneficently 

 regulated by the difficulty or ease with which the particular kind 

 of food upon which the bird subsists may be obtained. Thus, 

 large birds of prey which live upon flesh, such as eagles, vultures, 

 and falcons, usually lay two eggs, rarely more than three, and 

 frequently only one. Birds that live principally upon seeds and 

 insects, and thus find abundance of food, not only lay a larger 

 number of eggs, but often hatch two and three broods in the year 

 so that these families increase rapidly. 



961. Why do birds seleot silk, cotton, wool, fur, and down at 



materials for lining their nests ? 



Because these substances are non-conductors of heat ; and as it is 

 very essential that the nest should be kept perfectly warm, they 

 could not possibly select more appropriate materials for tho 

 purpose. 



962. In what manner do birds build and prepare their nests ? 



As the laying season approaches, the bird, conscious of the 

 coming event, occupies herself in the construction of a dwelling, 

 suited by its materials and form to the little beings to which she 

 is about to give life. 



Such a structure must fulfil several conditions. In its magnitude 

 and form it must correspond with the magnitude and number of 

 eggs to be laid, and with the body of the motfier who is to sit 

 upon them. 



It must be so shaped as to keep the eggs securely together, and its 

 materials must be soft, so as not to injure by undue pressure its 

 tender occupants. 



To prevent the escape, of the warmth imparted by the mother, 



