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In some cases, however, the effect of wear is quite 

 marked. Thus Beebe points out that in the cock 

 sparrow the throat feathers have dusky-brown tips, and 

 as these wear away in the spring the clear black centers 

 appear. Thus the worn sparrow is more handsomely 

 marked than the one which has recently moulted. A 

 very extraordinary case is that of the tropical American 

 motmot (Momotus), which has long tail feathers, the 

 ends racket-shaped, with the shafts bare for a con- 

 siderable distance before the broad tips. It is found 

 that the birds themselves remove the barbs for a con- 

 siderable distance, and thus produce this singular effect. 

 Are we obliged to suppose that these birds, like some 

 human beings, regularly mutilate themselves for the 

 sake of fashion ? It seems to be the case that for a 

 certain distance the barbs are loosely attached, and 

 hence fall away as the bird preens the tail feathers. 

 Thus it is possible that a structural peculiarity and an 

 instinct combine to produce the result, without any 

 deliberate intention on the part of the bird. 



5. The bird's bones are peculiar, yet they agree in Anatomy of 

 general type with those of other vertebrates. The birds 

 lungs are supplemented by a series of air cavities, and 

 even many of the bones in the majority of birds contain 

 air. In the ostriches and penguins, which do not fly, 

 there are no air spaces in the bones, but their presence 

 is not invariable in flying birds. The terns and swifts, 

 remarkable for their powers of flight, have solid bones. 

 The sternum or breastbone in flying birds is keeled, 

 presenting a more or less narrow edge extending out- 

 ward, as every one who has carved a chicken knows. 

 This keel affords attachment to the great pectoral 

 muscles, which are used in flight. The early experi- 

 ments in aviation, in which men attached winglike 



