32 EXTERNAL PARTS 



of tail feathers is almost always even, and varies from none to 

 upwards of thirty. This statement seems to indicate great 

 variation among birds with reference to the rectrices ; in 

 reality there is but little variation, as a very large proportion 

 of birds have twelve, and the numbers eight, ten, twelve, 

 and fourteen, will include all except a few odd forms, most of 

 which are not found in the region covered by this book. The 

 rectrices have their bases covered, both above and below, by 

 short feathers called upper tail coverts and lower tail coverts. 



Arrangement of rectrices. — The central pair of tail feathers 

 is above all the others, and each successive pair outward lies 

 under all the preceding ones. 



Forms of rectrices. — A tail feather of the English sparrow 

 illustrates the usual form. It can be seen to widen gradually 

 toward the tip. The more important variations from this 

 type are the lanceolate,^ where the vane is widest near the base, 

 and gradually narrows toward the tip ; the linear,^ where the 

 vane is narrow throughout; and the filamentous,^ where it is 

 very narrow, as in the outer tail feathers of the barn swallow. 



Varieties of tip and texture. — The usual tip is rounded;'' if 

 very abruptly and squarely tipped, it is said to be truncate; 

 if obliquely and concavely cut off, incised; if regularly sharp- 

 pointed, acute; if abruptly sharpened, acuminate.^ Most acu- 

 minate feathers are apt to be stiff and are used as an aid in 

 climbing ; such feathers are said to be rigid. Some feathers, 

 while having the vane rounded, have the rhachis extending as 

 a hard point beyond it ; in this case the feather is spinous '^ or 

 mucronate. 



If the vane, instead of having its margin straight, has 

 its edge in rounded curves, it is said to be cremdate. Some- 

 times the rhachis curves upward at the center ; in this case the 

 feather is said to be vaulted or arched. If the bending is side- 



