272 HANDBOOK OF NATURE STUDY 



other. This resemblance is due to the fact that all birds 

 have their fore limbs changed into wings, are covered with 

 feathers, and have a horny bill. Do you know of any birds 

 that cannot fly ? 



All birds lay eggs, and nearly all in nests more or less skil- 

 fully built by themselves ; only a few lay their eggs in the 

 nests of other birds. The eggs consist of a hard, calcareous 

 shell, the white, and the yolk, which is of a yellow or reddish 

 color. Every egg has a small space filled with air. 



The warmth of the brooding bird causes the young to 

 develop in the eggs. If during the period of incubation the 

 eggs are thoroughly chilled, the young birds in them will 

 die. The time necessary to hatch the eggs varies much 

 with the size of the birds. The young of most birds are 

 blind, naked, and helpless when hatched, and depend entirely 

 on their parents for food ; but the young of chickens, ducks, 

 and geese are covered with a soft down, can see at once, and 

 after a day or two they move about with the mother and 

 find their own food. 



Most birds breeding in cold or in temperate regions mi- 

 grate south on the approach of winter and return in spring ; 

 these are Migratory Birds. A bird that remains in the same 

 region winter and summer is called a Resident Bird. 



The most characteristic marks for the classification of 

 birds are found in the structure of the bill, the legs, and 

 feet. 



A very large group of birds whose feet are adapted to 

 perching on trees, and most of which have some power of 

 song, are frequently called Song Birds. 



A much better marked group is formed by ducks, geese, 

 swans, and similar birds. Their body is somewhat boat- 

 shaped ; their legs are short and are set well back ; the toes 

 are webbed and form powerful paddles. They can use their 

 bill as a strainer to separate small aquatic animals from the 



