274 Unversity of California Publications in Zoology  |Vor. 18 
be traced without a break in such a bird as Circus hudsonius, as 
has been done by me (1914). As was shown there, after the diminu- 
tion in number of barbs and reduction of barbules, as is the con- 
dition in ear coverts, the next step is the complete loss of the terminal 
barbs and elongation of the shaft into a bristle. Then follows the 
loss of more and more barbs and stiffening of the shaft, until the 
latter becomes a stout, unbranched bristle, as in the eyelashes of 
many birds. Usually rictal, supraorbital, and nasal bristles have 
some of the barbs still present; in the dense nasal tuft of Corvus 
the structure is very much like that of ear coverts, except that the 
barbs are set at a more acute angle with the shaft, thus producing 
narrower vanes. Comparatively few birds possess eyelashes, but 
when present they are so modified that in some cases nothing re- 
mains but the stout, deeply pigmented quill, totally devoid of any 
barbs or barbules. Such is the ease in hornbills, Geococcyx, and 
some other birds. In birds with aftershafts, although the main 
shaft is entirely bare, the former is represented by a few small, 
weak barbs with rudimentary barbules, e. g., Circus, Cathartes, and 
some others. 
7. Facial Ruffs 
In a few birds, e. g., owls and Circus, facial ruffs are developed, 
composed of several rows of closely grouped, very compact, curved 
feathers. The shafts are stiff and inserted almost at right angles to 
the surface of the body, only the tips beg curved so as to lie 
flat on the contour. The solid compact vanes are made so by the 
close approximation of the barbs to each other, and by the exceedingly 
numerous barbules which have well-formed and characteristic bar- 
bicels, but are short, due to the shortness of the individual cells, an 
obvious correlation with the close approximation of the barbs. 
IV. Cotor Propuction 
1. Isotely in Production of Colors 
The colors of feathers have been studied by a great many 
workers, chief among whom may be mentioned Altum (1854a, 1854b), 
Bogdanow (1856), Fatio (1886), Church (1893), Krukenberg (1882), 
Gadow (1882), and Strong (1902). As shown by the researches of 
these men and others, the colors of feathers fall into three cate- 
