I go THE WORLD OF ANIMAL LIFE 



to stay all the year round in one locality. They therefore migrate, 

 many of them travelling very long distances, and even crossing 

 very wide seas. Many of the birds we see in this country in 

 spring and summer leave for other lands in autumn. When it is 

 time for them to go you may see the different species gathering 

 in large flocks in your own district. As they cross the country 

 these are joined by other flocks until they look almost like black 

 clouds passing through the air. Other birds, which do not leave 

 the country, migrate from the fields and moors to the shore, or 

 seek shelter in woods and gardens. 



At first it seems very wonderful how birds, which are usually 

 such small creatures, should be able to cross wide seas in safety. 

 One would naturally expect them to fall into the water exhausted 

 long before they reached the other side. But the reader has per- 

 haps noticed, when travelling on a train that was running at a high 

 speed, that many birds are able to fly as quickly as the train, and 

 apparently with very little trouble. This is because the body of a 

 bird is so wonderfully shaped that the air offers very little resist- 

 ance to it, while the muscles with which it works its wings are very 

 powerful, and can drive it along swiftly for a long time without 

 getting exhausted. So strong are these muscles that a swan has 

 been known to break the oar of a boat with a blow from its wing. 

 It is by their great muscular wing-power, therefore, that birds are 

 able to travel such long distances. 



If you put your hand out of the window of a railway-carriage 

 you will find that the swift motion through the air makes it very 

 cold in a short time. How is it, then, that the rapid flight of birds 

 does not benumb their bodies, especially when they fly high up, 

 where the air is much colder than it is nearer the ground? It is 

 because they are so perfectly clothed with feathers, which keep in 

 the heat and keep out the cold. These feathers are the character- 

 istic distinction between birds and mammals. There is, of course, 

 the fact that they fly; but then all birds do not fly, while some 

 mammals do. There is also the distinction that they produce 

 their young from eggs; but, as we have seen, there is a mammal 

 in Australia which, besides having a bill like a duck, lays eggs from 

 which its young are hatched. But all birds have feathers, while no 



