XII THE COLOURS OF BIRDS 249 



logical Observatory, gives numerous instances of this 

 change occurring in birds, and distinguishes between 

 cases in which the colour-change is associated with 

 a change in feather-structure, and those in which it 

 is not so associated. Thus in the Hnnet and mealy 

 redpole, the surface of the barbs is said to peel off, 

 exposing the fresh and bright colours beneath. In 

 the case of the guillemot and little auks, on the 

 other hand, he describes an increase in the amount 

 of pigment in the feather without any textual 

 change. Gatke enumerates numerous other instances 

 of colour-change produced in this way by an in- 

 creased amount of pigment, or by a rearrangement 

 of pigment, as when black or blackish-brown replaces 

 white or gray, or when black and white replace 

 gray. 



A change of colour accompanied by a shedding 

 of part of the feather seems not inexplicable, but 

 the mechanism of a colour -change without this 

 is diiificult to understand, and the fact has been 

 strenuously denied by many. A recent summary 

 by Schenkling gives an account of the various 

 opinions which have been held on the subject. 

 Schenkling himself strongly inclines against the 

 view that a notable colour-change can occur without 

 a moult or a shedding of portions of the feathers. 

 He believes that the confusion has arisen from the 

 fact that the same moult has not identical effects 

 upon all the individuals of a species, or even upon all 

 the feathers of an individual. It is thus possible 

 to obtain specimens displaying feathers apparently 

 characteristic of successive moults. Such specimens 

 have been described as birds showing colour-change 



