THE BIRDS 415 



We do know, however, that the urge to migrate develops as a result 

 of the changing length of day that comes with the seasons. Suppose we 

 take a migratory bird in the north in midsummer and put him in a cage 

 where he receives light for only a part of each day. This artificial short 

 day causes the development of the urge to migrate, and when he is re- 

 leased from the cage he will start his flight south. Most birds feed only 

 when it is light and roost at night. Many come north for the summer, 

 because the days are longer in the north during the summer than they 

 are in the south. In winter, however, the reverse is true — the longer 

 days are to be found in the south. Long days are also correlated with 

 abundant plant growth, and the birds find abundant food the year around 

 when they migrate to the regions with the longer days. Poultrymen 

 sometimes take advantage of the habit birds have of eating as long as 

 it is light and keep a light on in their poultry houses. This causes the 

 chicken to feed almost continuously and they grow much faster and lay 

 more eggs than is possible when they spend about one half of their time 



roosting. 



Typical Birds 



There are about twenty-five orders of birds with many varied families 

 under most of the orders, so it will be impossible to give a complete sur- 

 vey of the field. However, by taking a few well-known birds and learn- 

 ing something about their habits, we may better understand the class. 



The Eagles. Perhaps it is appropriate that we start our study with 

 the group that includes the American national bird, the bald eagle. This 

 eagle is not really bald, but has a thatch of white feathers on its head 

 that makes it appear bald at a distance. It is a majestic bird, with a 

 wing spread of about six feet, that soars high in the air. only to drop 

 like a bullet when it spots its prey. It catches fish and some mammals 

 for food. Unfortunately, everything that can be said about our national 

 bird is not good. It is sometimes a robber. It will soar in the air 

 watching the osprey hawks. When the osprey dives in the water and 

 brings out a nice wriggling fish, the eagle will drop out of the sky and 

 attack the smaller osprey, causing it to drop the fish. The eagle then 

 drops down, catches the fish, and flies away. There are tales of eagles 

 carrying off small children and other large objects, but even the largest 

 of the eagles could not lift a weight of more than eight pounds. Eagles 

 build huge nests of sticks in rocky, inaccessible places in which they live 

 and raise their young year after year. 



The Owls. "Wise as an owl" is an old saying; it would seem that 

 any creature that looks so dignified and sits so long, apparently in deep 



