474 On Moult and Colour-change in Birds. 
This form of abrasion may be seen in the Linnets and Redpolls 
in late spring. Lastly, there is, I believe, a form of abrasion 
(of which it is very difficult to obtain positive proof) in which 
the outer layers of the sheath of the feather become rubbed 
off, allowing the pigment to show through more clearly ; if 
this is so, the brighter colours of most birds at the approach 
of spring may be accounted for in this way. M. Fatio, in 
the paper cited above, is of opinion that oil found its way 
up and diluted the pigment, thus bringing it nearer the 
surface ; but, from a microscopical examination of the 
feathers, the granules of pigment in the brighter feathers do 
not appear larger than, or in any way different from, those in 
the duller feathers. This abrasion as a moult, and consequent 
change of colour, must be carefully distinguished from the 
ordinary abrasion, caused by wear and tear of the feather. 
The first, takes place about a regular time and is of com¬ 
paratively short duration, and the abraded feathers have even 
edges and resemble newly-grown feathers. In the other 
case the abrasion may take place at any season, the abraded 
edges are always irregular, and if there is any change of 
colour, it is in the form of bleaching. While dealing with this 
subject, passing allusion must be made to a valuable paper by 
Dr. Gadow* on metallic colours and how they are caused, 
in which he points out how the metallic colours are due to 
the structure of the outer sheath of the feathers, which act 
as so many prisms. Bearing this in mind, it would not 
be surprising to find that a complete change of colour may 
be brought about by an alteration in the structure of the 
outer sheath of the feather, the structure being altered by a 
total or partial abrasion. 
Lastly, we have an alteration or re-arrangement of pigment 
in the fully-grown feather, and probably in some cases an 
influx, concerning which enough has already been said for 
the present. 
* “ On the Colour of Feathers as affected by their Structure,” by 
Dr. Hans Gadow (P. Z. S. 1882, p. 409). 
