THE COLOURS OF BIRDS 



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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- 



FiG. 4. — Metallic barbs from feathers of humming-birds, 

 magnified, a is from a feather belonging to a female of 

 Eustephanus femandensis, 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 nalced. 



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 



