i6 THE BIOLOGY OF BIRDS 



street we see horses turning round the pinna of their ear on 

 occasions when we should adjust our whole head. 



The development of skin-outgrowths to form combs 

 and wattles and similar structures will be discussed in 

 connection with sex, for they are usually most marked in 

 the male bird. 



One of the most peculiar external characters, which has 

 been much discussed without great result, is the serrated 

 or pectinated claw of the nightjar. It also occurs in the 

 bittern, gannet, heron, and courser. Perhaps it has to do 

 with some sort of preening. Like various peculiarities in 

 beaks, as in the Skimmer, the pectination is not found in 

 the nestling, but develops later. This is decidedly against 

 one of the theories, that the pectination is a vestigial structure 

 which has lost the function it had in some bygone ancestor, 

 and is no longer of any importance. 



§ 3. Plumage of Birds 



Uniqueness and Specificity 0! Feathers. — A bird is 

 known by its feathers. This famiUar statement expresses 

 two facts — (a) that feathers are strictly confined to birds , 

 and (b) that different types of birds have different types of 

 plumage, (a) Although a feather is like a scale in being 

 an epidermic structure and in its simplest forms may be 

 thought of as the frayed-out base of a scale, there is no 

 connecting link between scale and feather, and, as we have 

 seen, the evolution of feathers is quite obscure, (b) It has 

 been shown by Chandler (191 6) and others that relationships 

 between birds, as indicated by skeletal and other parts, are 

 often corroborated by characteristics in the structure and 

 distribution of the feathers. The important points are in 

 the aftershaft, the barbs, the distal barbules of the pinions, 

 and the degree and manner of simplification seen in the 

 barbules of the body-feathers. Since the number of 

 minute structural units composing a feather is large there 

 is more opportunity, as it were, for the expression of 

 specificity than there is in the scale of a fish. Moreover, 



