140 



Diagram showing (1) section of barbs (rami) and (2, 3) interlocking 

 barbules (radii). (After Pycraft.) 



tail, in birds which possess full powers of flight, are always well- 

 developed and conspicuously large (p. 139). 



Semiplumes are degenerate contour-feathers and have discontinuous 

 vanes. Sometimes they are of great size and beauty and are accordingly 

 much prized for decorative purposes. For instance " Marabou" feathers 

 are the semiplumes of the Marabou and Adjutant-Storks. 



Down-feathers are very delicate in structure, the shaft, when present, 

 being very short and weak, whilst the barbs are long and fragile. 

 They are almost always hidden below the contour-feathers, but are 

 occasionally exposed and they form the ruff round the neck of the 

 Condor and certain Vultures. In water-birds these feathers form a 

 thick underclothing recalling the under-fur of Mammals, and often, 

 as in the Swans and Eider-Ducks, have a considerable commercial 

 value. Some birds, such as the Grame-birds, Pigeons, and Hornbills, 

 have no down-feathers. 



Filo-plumes are long hair-like feathers bearing a minute vane at the 

 tip, and occur in clusters round the base of the contour-feathers. In 

 some birds, for instance in the Cormorants, they appear on the surface 

 of certain parts of the body, notably on the head and neck. 



Powder-down feathers occur only in a few groups of birds, either 

 sparsely scattered over the body, as in Parrots and certain Hawks, or in 

 patches on the breast and thighs, as in the Herons. They are remark- 

 able for their extreme friability, constantly breaking up at their tips 

 into a fine powder, which feels smooth and almost greasy to the touch. 

 Nothing is known concerning the development or use of these extra- 

 ordinary feathers, 



The long stiff bristles, which occur round the mouth of certain birds, 

 such as the Nightjars, or form eyelashes in others, for instance in the 

 Ostrich and Ground- Horn bill, and the peculiar tuft which hangs from 

 the breast of the Turkey, are degenerate contour-feathers, which have 

 lost their vanes. 



