178 BIRD WORLD. 



When you read about feathers, you learned that all 

 birds had feathers, but that there were some who could 

 not fly. I think you have now learned enough about 

 flight to see why this is so. The heavy birds with 

 short winofs — the Auk and the Ostrich — cannot 

 support such weight in the air, so must get along 

 with swimming and running and diving. The long- 

 winged swallow and the broad-winged hawk are as 

 much at home in the air as the fish is in the water. 



FLIGHT. 



Have you ever wondered why it is that a bird 

 flies so surely and straight where he wants to go, 

 while a butterfly flits about in such a haphazard way ? 



Those of you who have had to do with boats will 

 know what ballast is, and how necessary it is to a 

 boat's even, steady progress. The weight of the boat 

 should be well down in the water. The bird is like 

 a well-ballasted boat. The heavy muscles and the 

 stomach, with its weight of food, are all in the " hold," 

 so to speak, — all down as low as possible, — and the 

 expanse of the wing is not great enough to out- 

 balance this. In the case of the butterfly the wing 

 expanse is so great and the weight of the body so little 

 that the insect flutters about, driven out of its course 

 by every breath of air. 



