276 
COLOUR IN NATURE 
CHAP. 
Structure of Metallic Feathers. — The brilliant 
metallic feathers of the head region of many male 
humming-birds are in several respects very peculiarly 
modified. They are very short, much rounded, and 
overlap one another ; the surface is strongly metallic 
and marked with deep ridges (see Fig. 3). A further 
point of interest is that the barbs, quite devoid of 
barbules, are prolonged as a delicate fringe beyond the 
apex of the feather. While for further details I must 
refer to my paper on the subject, we may simply notice 
that the metallic colouring is here not produced by a 
modification of the distal portion of the barbules, but 
by a deeper pigmentation and a structural change in 
the proximal region. The result of this is that the 
metallic colour in humming-birds tends to appear 
first in the middle region of the body feathers—that 
is, the region where the barbules tend to attain their 
maximum development, and not at the tip of the 
feathers as in sun-birds. This primitive condition is 
well seen in the breast feathers of the female of 
Eustephanus fernandensis , and of both male and 
female of E. galeritus. Here we have white or dull- 
coloured feathers, with a central spot deeply pig¬ 
mented, and displaying a varying amount of metallic 
colour. The increasing specialisation of the metallic 
region is accompanied by a gradual retrogression of 
the apical region which is eventually represented 
only by the slender naked barbs. 
The metallic modification of the feathers in 
humming-birds is therefore not accompanied by 
any change which affects the locking together of 
the barbules, and so the adaptability for purposes 
of flight; it differs in this respect sharply from the 
