AVES 



357 



77ietallic colors are due to the structure of the surface of certain parts 

 of the feathers such as striae^ ridges, knobs or pits, in combination 

 often with an extremely colorless layer, these elements acting as 

 prisms. Dr. R. M, Strong believes that in the pigeon the metallic 

 colors of the neck feathers are due to spherical granules of the trans- 

 parent wall and terms them " plate interference colors " or New- 

 tonian rings. Feathers are found only on certain feather tracts 

 which differ in different species of birds. 



Fig. 200. Foot forms, a, semi-palmate, wading of Ciconia; b, perching of Turdus; 

 c, rasorial of Phasianus; d, raptorial of Falco; e, adherent of Cypselus; f, cursorial of 

 Struthio; g, zygodactyl (scansorial) of Picus; h, lobate of Podiceps; i, lobate and 

 scalloped of Fulica; k, palmate of Anas; I, totipalmate oi Phaethon. (From Schmarda. 

 Hertwig-Kingsley. Courtesy of Henry Holt & Co.) 



Skeleton. — The forelimbs and pectoral girdle (Figure 201) are 

 modified for flight. The skeleton of the limb is rigid. The hind 

 limbs and pelvic girdle are used for bipedal locomotion. The 

 skeleton is permeated by air usually in ratio to the mode of life. 



(i) Condors and cranes, soaring birds, have lightly built skele- 

 tons. (The snipe and the curlew have airless bones, but fly long 

 distances.) 



(2) Ducks and other water fowls have cavities of the long bones 

 filled with marrow. 



