50 Bird - Lore 



fills the bones of mammals is absent, the bones are hollow, and their walls, 

 moreover, are thin. Here is another great saving in weight. They are, likewise, 

 connected by air-sacs with the lungs and filled with air. These air-sacs fill 

 every available space within the body of the bird that is not occupied by some 

 organ and when they are all filled with air the bird becomes much lighter for its 

 size. 



A RLACK TERN AT ITS XEST, ILLUSTRATING THE LONG, 

 NARROW WING ADAPTED TO GLIDING 



Examine a bird's mouth and one finds not the slightest vestige of a tooth. 

 The lirst birds had them but they all disappeared ages ago because they were 

 heavy and required heavy jaws to support them and heavy muscles to manipu- 

 late them. A real, efficient Hying bird could not have all this extra weight, so 

 Mother Nature devised another method of grinding the bird's food and today 

 it is done in the stomach, which is called the gizzard, and the bird's head 

 weighs but little. 



The first bird, likewise, had a long lizard-like tail with feathers along the 

 sides but this, too, was heavy and so graduall}- it became shortened until today 



