SEC. Ill 



EXTERIOR PARTS OF BIRDS 



171 



that draw on either side upon the bases of the quills collectively ; 

 they are the same that pull the whole tail to one side or the other, 

 acting like the tiller-ropes of a boat's rudder. The general 



Shape of a Reetrix is shown in Fig. 23. Such a feather is 

 ordinarily straight, somewhat clubbed or oblong, widening a little, 

 regularly and gradually toward the tip, where it is gently rounded 



Fio. 32.— The Lyre-bird of Australia, Mmura superla, to show tbe unique lyrate shape of 

 the tail. 



off. But the departures from such shape, or any that could be 

 assumed as a standard, are numberless, and in some cases extreme. 

 In fact, none of a bird's feathers are more variable than those of 

 the tail; it is impossible to specify all the shapes they assume. 

 While most are straight, some are curved — and the curvature may 

 be to or from the middle line of the body, in the horizontal plane, 

 or lip and down, in the vertical plane. Some shapes have received 



