78 



THE FLIGHT OF BIRDS 



other big, long-winged birds have wing-bones of 

 larger girth (of a girth, that is, disproportionate to 

 their superiority in length of wing) in order that 

 they may be able to bear the far greater strain put 

 upon thern. A proportionate increase in weight 

 would, no doubt, have caused difficulty, and this 

 has been obviated by the aeration of the bones. 



Fig. 22. 



Humerus of (1) Skua; (2) Rhinoceros Hornbill ; (3) Sea Eagle 



— actual length 6f inches. Drawn to scale. F : Foramen, where 



the bronchial membrane enters, on the upper surface. 



To bring out this point more clearly I have taken 

 the humerus of the Skua as the standard, and have 

 calculated what would have been the length of the 

 same bone in the Sea Eagle if it had been built on 

 the same lines. 



After all this I have to admit that there remains 

 a very puzzling case on which I cannot throw any 

 light. The Hornbills are slow, heavy flyers, they 

 are not very big, and they are the most pneumatic 



