274 University of California Publications in Zoology fVoL. 13 



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 Corviis 

 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 case 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 being 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. Color Production 

 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- 



