1916] Chandler: Structure of Feathers 273 
cases are the barbules lost, except in connection with color pro- 
duction, or in case of the tips of the barbs being transformed into 
stiff spines as in the tails of woodpeckers. A farther modification 
found in contour feathers is the formation of a terminal undivided 
horny expansion, produced either by the shaft alone, as in certain 
rail feathers (Bonhote, 1912), by the coalescence of the shaft and 
both vanes, as in the crown feathers of the curly-headed toucan 
(Pteroglossus beauharnaist), or by the fusion of the shaft with 
the terminal portion of only the outer vane, as in the 
of Bombycilla garrula (pl. 33, fig. 95a). 
wax tips”’ 
5. Har Coverts 
On the head of most birds there are a number of modifications 
of contour feathers to serve special functions, and they are won- 
derfully adapted to serve their particular purpose. 
First among these may be mentioned the ear coverts. The typi- 
cal structure of these feathers is similar to that described by me 
(1914) in Circus hudsonius. They are loose-vaned feathers, with 
the barbs wide apart on the shaft, and the short, awl-shaped, degen- 
erate pennaceous barbules closely appressed to the barbs, thus pro- 
ducing a mechanism admirably fitted to catch dust particles and 
yet not obstruct sound. In birds which have well-developed after- 
shafts this structure is also present in the ear coverts, but in much 
modified form. In the Limicolae, herons, hawks, and some others, 
the aftershaft is greatly developed and almost equals the main 
feather in both size and structure. In Tetraonidae the aftershaft 
is reduced to a very small downy pad, scarcely larger than a pin- 
head, which takes no part in covering the ear. An intermediate 
condition occurs in Grus, where the basal portion of the aftershaft 
is densely downy, while a few of the barbs are elongated, with the 
typical appressed, inconspicuous barbules. In owls and many other 
coraciform and many passerine birds, the ear coverts lack an after- 
shaft entirely. As has previously been pointed out (Chandler, 1914), 
the ear coverts are undoubtedly adaptive modifications of contour 
feathers, which are in a transitional stage of transformation ulti- 
mately leading to the various kinds of facial bristles and eyelashes. 
6. Facial Bristles and Eyelashes 
The steps in transformation from ordinary contour feathers of 
the trunk to the highly modified eyelashes of certain birds may 
