74 VERTEBRATES: HIRDS. 



and vane on each side, the hatter being the broad portion 

 which consists of dehcate plates that are united by 

 minute barbs along their edges, and thus made firm, — 

 the plates not separating from one another when pressed 

 against the air, as in fl)'ing. There are, however, downy 

 feathers on every Bird, or such as do not have the plates 

 united. The plumage of Birds is made waterproof by 

 the oil with which they dress their feathers, and which 

 is obtained from a gland situated on the tail. They 

 shed their feathers twice a 3'ear, and in many kinds the 

 winter plumage differs in color from that of the sum- 

 mer. 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 recei\'ed 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. In 

 seed-eatin<j Birds, the gizzard contains gravel and other 

 hard substances, \\'hich these animals swallow to aid 

 the gizzard in grinding the seeds. 



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

 most Birds build nests in \\hich 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 \\-hich it breaks the shell. This point is plainly 

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

 da)-s it falls off. 



The number of kinds of Birds is about ten thousand, 

 and there are about se\en hundred kinds in North 

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

 the Seratchers, the Runners, the Waders, and the 

 Swimmers arc the large groups into which Birds are 

 divided. 



