264 THE BIRDS 



laminae known as barbules, which serve to interlock the barbs, 

 forming a surface which is able to withstand the rush of air, and 

 is practically impervious to water. The shaft and the barbs 

 together form the vane or vexillum of the feather. The contour 

 feathers of the majority of birds bear two vexilla ; the second, 

 termed the after-shaft, is usually a smaller and more delicate edition 

 of the main shaft, from the base of which it springs, but in some 

 species of the ostrich tribe and a few nestling birds the main shaft 

 and the after-shaft are equal in size. 



The down feathers differ from the contour feathers. The 

 barbs are very long, soft and fragile, and are deficient in the inter- 

 locking barbules. In many cases, instead of being arranged in a 

 double row down each side of a shaft, the barbs spring in a tuft 

 from the top of the quill. When the shaft is present it is short 

 and weak. Game birds, pigeons and hornbills have no down 

 feathers. 



Powder-down feathers, which occur in tufts or patches in certain 

 birds of prey, in parrots, and more especially in the heron tribe, 

 are down feathers of an extremely friable nature, which are con- 

 stantly breaking up at the tips into fine powder, often imparting a 

 curious bloom to the plumage of the bird. 



Filo-plumes, or thread feathers, are long, hair-like feathers 

 growing in small clusters at the base of the contours. In some 

 birds they grow very long and project in tufts beyond the contours. 

 In the cormorants these filo-plumes show as large white patches on 

 the neck and thighs. 



A bird's feathers, except in the penguins, are not evenly dis- 

 tributed over the entire surface of the body, but grow in well- 

 defined rows or tracts with bare spaces in between them, which are, 

 however, in most birds covered with down. The contour feathers 

 covering the head and body overlap one another much in the 

 same way as do the scales of fishes and reptiles; and the large 

 flight feathers which fringe the hinder border of the wings are 

 arranged in such a manner that, when the wings are extended, 

 the feathers overlap one another, laterally, and the free edges face 

 outwards. 



The varied colours of the feathers, to which many birds owe 

 their wonderful beauty, are due either to the presence of actual 

 pigment, to pigment overlaid with colourless structures, or to 



