18 THE STORY OF BIRD-LIFE. 



The reason for the existence of these tracts is 

 not so obvious. It has been suggested that they 

 represent the regions of the body where resistance 

 during flight is least. That is to say, their re- 

 striction to certain areas, gives greater freedom 

 of movement to the limbs. But this is not a 

 very convincing explanation. Possibly we should 

 be nearer the truth to regard it as an instance 

 — one of many — of the economy of material so 

 often practise! by Dame Nature. 



The number of feathers, or rather the size of 

 the tracts bears some sorb of relation (a) to the 

 amount of specialisation which the bird has under- 

 gone generally ; and (b) to the particular mode 

 of life which the bird follows. Thus, the most 

 highly specialised of all birds — the passeres, of 

 which a crow may stand as the type, are the most 

 scantily clothed of birds, the tracts being very 

 narrow, and the spaces very wide, whereas the 

 birds of the ostrich kind have the feather tracts 

 of great breadth, so much so, as to make it diffi- 

 cult to find the spaces. 



So far, the feathers of which w^e have been 

 speaking have been those known as the contour- 

 feathers ; because, as their name implies, they 

 form the contour or outline of the body. In many 

 birds, as a duck, for instance, if we raise the 

 contour-feathers, we find below a thick under- 

 clothing of down-feathers. These will need no 

 lengthy description. They are familiar to all, 

 either in the shape of eider-down or swan's-down. 

 They are found to be closely interspersed amongst 

 the bases of the contour-feathers, and over the 

 spaces, so as to form a thick underclothing. 



