XIII 
THE COLOURS OF BIRDS 
277 
modification seen in the sun-birds where the metallic 
barbules are entirely unconnected. We can thus 
understand how it is that the quills of humming¬ 
birds may display structural colour without their 
efficiency being in any way impaired. The fact is 
also readily explicable on structural grounds, if we 
recollect that it is the mid region of the feather 
which tends to become metallic, and it is this region 
which is most fully developed in quills. In humming- 
a b 
Fig. 4.—Metallic barbs from feathers of humming-birds, 
magnified, a is from a feather belonging to a female of 
Eustepha. 7 ius fernandensis, the barbules at the base are 
metallic, but the barb also bears rudiments of barbules at 
its tip. b, a barb from the feather shown in Fig. 3 ; all the 
barbules are metallic and the tip of the barb is naked. 
birds the colour differences between the male and 
female, or between specialised and unspecialised 
species, are thus largely the result of an increased 
amount of melanin pigment in the brilliant forms, 
accompanied by a process of structural modification. 
The type of metallic colour seen in the humming¬ 
birds is of much interest, because it has not been 
described outside of the family. In other cases we 
