56 



THE BIOLOGY OF BIRDS 



ranked as an adaptation, for it affords insertion to the tail 

 feathers or rectrices, which can be used as a brake in flight, 

 or by being suddenly raised or lowered may enable the 

 bird to avoid a telegraph wire or the like. The actual 

 steering in a flying bird is effected by the differential action 

 of the wings and in some measure by tilting the body by 

 means of the wings. Anatomically, the pygostyle may be 



Fig. 10. — Pelvic girdle and sacral region of an ostrich. From a 

 specimen, il.-il., beginning and end of the ilium ; is., ischium ; p.pr., 

 pectineal process ; AC, acetabulum; p.p., post-pubis ; p.s., pubic sym- 

 physis, peculiar to the ostrich ; s,v., sacral vertebrae ; ss., syn-sacrum ; 

 C.V., free caudal vertebrae. 



distantly compared with the coccyx, a post-sacral fusion of 

 vertebrae in man and anthropoid apes, and, also remotely, 

 with the urostyle or post-sacral fusion of vertebree in the 

 frog. It is interesting to find that the ploughshare bone is 

 particularly large in birds with strong tail feathers. This 

 may be well seen in the woodpecker, where the tail feathers 

 serve as a prop in climbing a tree. This illustrates functional 

 adaptation. But it does not follow that the usage of the 

 feathers made the pygostyle large. 



