60 VERTEBRATES: BIRDS. 



on every bird, or such as do not have the plates united. 

 The pjumage of Birds is made water-proof by the oil 

 with which they dress their feathers, and which is ob; 

 tained from a gland situated on' the tail. They shed 

 their feathers twice a year, and in many kinds the win- 

 ter plumage differs in color from that of the summer. 

 In most birds the colors of the male are much more 

 brilliant than those of the 'female. 



Birds swallow their food without chewing it, and it is 

 first received into a sack called the crop ; then it passes 

 into another sack, where it is moistened and softened ; 

 then it passes to the gizzard, where it is digested. The 

 gizzard generally contains gravel and other hard sub- 

 stances, which these aninjals swallow to aid digestion. 



Birds lay eggs and sit upon them to hatch them, and 

 most birds build nests in which to rear their young, 

 those of the same kind building alike. The young bird 

 in the egg has a horny point at the end of the bill, with 

 which it breaks the shell. This point is plainly seen 

 on the bill of the newly-hatched chicken ; in a few days 

 it falls off. 



The number of kinds of Birds is ten or twelve thou- 

 sand, and there are about seven hundred kinds in North 

 America. Birds of Prey, the Climbers, the Perchers, 

 the Scratchers, the Suntiers, the Waders, and the Swim- 

 mers are the large groups into which Birds are divided. 



BIRDS OF PREY, OR RAPTORES. 



These are the Vultures, Eagles, Hawks, Falcons, and 

 Owls. Most of them capture birds and other animals 

 for food. They are mostly of large size, and have a 

 f-trong hooked bill, sharp claws, great spread of wing, 

 aiid very powerful muscles, and the females are gener- 



