EXTERNAL FEATURES 21 



that there is no fat present, as is sometimes alleged. The 

 greasy feeling is due to the mechanical nature of very flexible 

 minute platelets of horn. 



What are called tactile feathers or vibrissae occur at the 

 root of the bill and round the eyes, and are best developed 

 in nocturnal birds. According to Kiister (1905), they owe 

 their tactility to touch-corpuscles at their base ; the nerve- 

 fibres that enter the papilla have only a vaso-motor 

 significance. 



Uses of Feathers. — (i) There can be little doubt that 

 the primary use of feathers is to form a relatively non- 

 conducting covering which tends to retain the animal heat. 

 This value may be increased in very cold environment by 

 the quality of whiteness, as in ptarmigan and snowy owl. 



(2) Secondarily the feathers made flight possible. As 

 adaptive to this may be noted the light elastic build of these 

 horny structures, the close almost air-tight linkage eflPected 

 by the barbules and the booklets of the pinions, and the 

 relatively great length often attained, far exceeding that of 

 the bones which bear them. 



(3) The feathers may help to render the bird incon- 

 spicuous by their close resemblance to habitual surroundings. 

 Thus the brooding woodcock among the fallen leaves and 

 withered herbage has what may be called a garment of 

 invisibility, and the white winter plumage of the ptarmigan 

 very eflPectively hides the bird among the snow. 



(4) The feathers are often auxiliary to the appeals made 

 in courtship activities. They may be of such a structure 

 that they enhance the brilliance of the bird as it moves 

 about ; they may form elongated decorations as in the 

 streamers of some Birds of Paradise ; they may be erected 

 in excitement, as is familiarly seen in pigeons ; they may be 

 used to produce sounds which arrest attention, as in the 

 " drumming " snipe. 



(5) In a great many cases, reaching a climax in eider-duck 

 and long-tailed-tit, feathers are used in the formation 

 of the nest, and not only conserve the heat around the 

 imperfectly warm-blooded nestlings, but make the brooding 



