STRUCTURAL ADAPTATIONS 399 



There is one other point concerning the Woodpeckers and 

 their use of the tail as a support which must be mentioned here, 

 and that is the fact that the pygostyle, or fusion of vertebrae which 

 terminate the tail skeleton, has its under-surface expanded to 

 form a large disc for the purpose of affording increased support 

 to the tail feathers. A similar support turns up again in the 

 Falcons, and these birds use the tail for the purpose of checking 

 their career when striking at their prey in mid-air, but in this 

 case the disc is formed by a pair of plate-like ossicles and not by 

 the expansion of a mass of bone. 



