350 University of California Publications in Zoology [Vol. ii 



RECTRICES 



Next to the remiges, the most highly developed feathers of the 

 body are those of the tail, which are of prime importance in 

 steering and balancing. They are commonly known as reetrices. 

 Circus hudsonius possesses twelve of these feathers, growing in 

 an arc from the posterior portion of the pygostyle, on the dorsal 

 side. Their bases are covered dorsally by the upper tail coverts, 

 and ventrally by the under tail coverts. 



The general structure of the shaft and vanes is very similar 

 to that of the primaries, except that the shaft is more nearly 

 straight, and not so wide and deep. The two middle tail feathers 

 have straight quills bearing two subequal vanes. The barbs are 

 set at an angle of about 40 degrees with the shaft, and are 35 to 

 40 millimeters long and 0.6 millimeter deep at the base. Proxi- 

 mally there are twenty-six per centimeter on each side, gradually 

 decreasing to fourteen per centimeter at the tip. The minute 

 structure of the middle reetrices is peculiar in that the proximal 

 barbules of both vanes have barbicels on the distal portion of 

 the barbs. The distal barbules of both vanes resemble those on 

 the inner vane of. the remiges. 



In the lateral reetrices there are a number of modifications. 

 The inner vane, which in the middle two feathers is 20 milli- 

 meters wide, in all the other tail feathers is about 25 millimeters 

 wide, while the outer vane steadily decreases laterally from 20 

 millimeters in the middle feathers, to 10 millimeters in the outer- 

 most one. As stated above, the tail feathers are arranged in the 

 form of an are, so that the outer ones are inserted at a wider 

 and wider angle to the longitudinal axis of the bird's body. 

 Unless some provision were made against it, this would result 

 in the tail being in a permanently spread condition. This 

 difficulty is overcome by an increasingly pronounced bend in the 

 quill near its base, so that the outside feathers, though inserted 

 at a wide angle to the long axis of the body, come to lie parallel 

 with the more central ones. The barbs of the inner vanes of the 

 feathers, which decrease in number from twenty-eight to thirteen 

 per centimeter from base to tip, possess proximal barbules of the 

 normal type only, none of them having distinct barbicels; they 



