CHARACTERISTICS OF BIRDS. rT 
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 sufficient 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 Hider-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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