738 



EVOLUTION OF ANIMALS 



Part V 



Fig. 36.9. Process of nest-building by a weaverbird (Quelea). The arrows 

 show the direction in which the string is pulled. A, the points of holding by the 

 beak. 4, 5, and 6 show stages of cooperative work by foot and beak. Weaverbirds 

 are relatives of house sparrows that range through Europe, Africa, and Australia. 

 Birds follow a set pattern of nest building. Weaverbirds raised by hand for four 

 generations made perfect nests of a type they had never seen. (After Friedman. 

 Courtesy, Young: The Life of the Vertebrates. Oxford, England, Oxford Uni- 

 versity Press, 1950.) 



above the water surface, and four long toes that distribute the weight on the 

 mud. The plover runs along the beach, swims into the waves, then scurries onto 

 the sand again on front toes that are partly webbed but still flexible for run- 

 ning. In ducks, geese, swans, and other water birds, the three front toes 

 are joined together by a web of skin that is outspread against the water in the 

 backstroke. When pulled forward the foot slips through the water, with toes 

 drawn together and webs folded. An ostrich runs on its third toe, the large 

 powerful one which supports most of its weight. The fourth or outer toe is the 

 small helper with a toenail only as large as the claw of a chicken. 



Internal Structure 



Modifications for Flight. Skeleton and Muscles. The skeleton of a bird 

 is modified for flight, for walking and running, for perching and for laying eggs 

 with hard shells (Figs. 36.10, 36.12, 36.13, 36.14). It bears several reminders 

 of the skeletons of reptiles. 



The skeleton is light yet rigid. Except in running birds, the bones of the 

 skull unite early making it strong against shocks, such as those from a wood- 

 pecker's hammering. The importance of vision in birds is emphasized by the 

 large eye sockets. The neck is commonly long, 14 vertebrae in a pigeon, 25 

 in a swan, with peculiar joints that allow the bird to turn it freely when watch- 



