FLIGHT OF BIRDS. 583 



butes to the ascent of the body on the same 

 principle that a kite is carried up into the air 

 when retained in an oblique position. This cir- 

 cumstance is particularly observable in the ascent 

 of birds of prey, whose wings have a great obli- 

 quity, and, when fully expanded, present a very 

 large extent of surface. 



The actions of the tail, which operates as a 

 rudder, are useful chiefly in directing the flight. 

 When the tail is short, this office is supplied by 

 the legs, which are in that case generally very 

 long ; and being raised high and extended back- 

 wards in a straight line, are of considerable 

 assistance in the steerage of the animal. In 

 many birds, as in the wood-pecker, the tail is 

 much employed as a support to the body in 

 climbing trees. The caudal vertebra? are often 

 numerous, but are short and compressed toge- 

 ther ; they are remarkable for the great deve- 

 lopement of their transverse processes, and for 

 having spinous processes both on their lower 

 and upper sides. The last vertebra, instead of 

 being cylindrical, has a broad carinated spine 

 for the insertion of large feathers. 



Birds could not, of course, be always on the 

 wing; for a great expenditure of muscular 

 power is constantly going on while they support 

 themselves in the air. Occasional rest is neces- 

 sary to them as well as to other animals, and 

 means are accordingly provided by nature for 



