THE ANIMAL SERIES 391 



for the opportunity to bite. Blowing vipers make a 

 hissing noise as a warning to an approaching enemy. 



The economic value of reptiles is not very great. Snakes 

 devour a great many bugs, some insects, and many field 

 mice, which are harmful to the crops. Turtles serve as 

 food for man. The skin of alligators is used for making 

 various useful leather articles. 



271. Birds. The relation of birds to reptiles can be 

 observed by noticing the scales on their feet and by com- 

 paring their eggs with the eggs of reptiles. The principal 

 difference between the eggs of birds and those of reptiles 

 is that birds' eggs have a hard, limy shell on the outside 

 and a tough membrane inside, while the eggs of reptiles 

 have only a tough but flexible membrane for a covering. 

 Some fossil remains of birds have also been found which 

 give evidence that birds originated from reptiles. 



Some birds are more destructive than beneficial; for 

 instance, the English sparrow, the crow, and the blackbird. 

 The English sparrow was introduced into the United 

 States in 1850, at Brooklyn, N. Y., in order to destroy the 

 insect known as the cankerworm, but it soon changed its 

 habit of living upon insects to that of living upon grain and 

 seeds, and thus became a menace to the welfare of the in- 

 sect-eating birds instead of being an insect-destroying bird 

 itself. It is able to adapt itself to all the conditions of its 

 environment. It does not migrate, but during the winter 

 finds shelter in and around buildings. In the spring the 

 English sparrow builds its nest, which is very large; the 

 inside is lined with soft feathers; and in it four to six eggs 

 are deposited. The female incubates the eggs until 

 hatched and then the parent birds start to feed the 

 young. Immediately the female again deposits eggs in 

 the nest with the young sparrows, which now incubate the 



