CHARACTERISTICS OF BIRDS. 121 



living in the tropics, as the Ostrich. The popular story 

 about this bird is not true. There is no neglect on her 

 part when she leaves her eggs in the sand, for when she 

 is in a temperate climate, where the heat of the sun is 

 not suificient to hatch them, she sits on them. The 

 Mound birds of Australia have a singular way of pro- 

 viding for heat in hatching their eggs. Instead of sit- 

 ting on them, they place them in mounds of decaying 

 vegetable matter, which they heap up for this purpose. 

 The process of decay produces all the heat that is requi- 

 site. Most birds make nests, not to live in, but to hatch 

 their eggs, lining them commonly with some soft mate- 

 rial. The Eider-duck lines her nest with down which 

 she strips from her own breast. 



206. The formation of a feathered animal from the 

 simple contents of an egg by the stimulus of heat is one 

 of the most wonderful things in nature. When the bird 

 is fully formed, it cuts its way out of the shell with an 

 instrument furnished it for this purpose, a pointed scale 

 fastened to the end of its beak. Any one can readily 

 see this on the upper bill of the newly-born chicken. 

 Soon after its birth this scale drops off, as the chicken 

 has no farther use for it. 



207. The senses which are most developed in birds 

 are the sight, smell, and hearing. The sense of touch 

 in most of them is very slight ; but some, as the Duck 

 tribe, have quite an acute sense of touch in their bills, 

 guiding them in their search for food. The sense of 

 taste is also, in most birds at least, very slight. The 

 sight is generally acute, especially in birds of prey* 

 Birds have a kind of third eyelid inside of the others? 

 called the nictitating or winking membrane. It is very 

 thin, and is commonly folded up in the corner of the eye 

 out of sight, but it can be drawn over the whole front 

 of the eye when it is needed. The bird can see through 

 it, and the object of it is to diminish the light that en- 

 ters the eye when it is very intense. It is this which 



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