FLIGHT OF BIRDS. 515 



the same principle that a kite is carried up into the 

 air when retained in an oblique position. This 

 circumstance is particularly observable in the ascent 

 of birds of prey, whose wings have a great obliquity, 

 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 assist- 

 ance in the steerage of the animal. In many birds, 

 as in the Woodpecker, the tail is much employed 

 as a support to the body in climbing trees. The 

 caudal vertebrae are often numerous, but are short 

 and compressed together: they are remarkable for 

 the great developement of their transverse pro- 

 cesses, 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 them- 

 selves in the air. Occasional rest is necessary to 

 them as mcII as to other animals ; and means are 

 accordingly provided by nature for their mecha- 

 nical support and progressive motion while on land. 



The anterior extremities having been exclu- 

 sively appropriated to flight, and constructed with 

 reference to the properties of the atmosphere, the 

 offices of sustaining and of moving the body along 

 the ground must be entrusted wholly to the hind 



