471 On Mdull 0)1(1 CoIoiir-rfKUHic in Birds 



This form of abrasion may be scon in the Linnets and JvcdpoUs 

 in late spring. Lastly, there is, I believe, a form of abiasion 

 (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. ]\L Fatio, in 

 the pa])er cited above, is of o[)ini()n that oil found its way 

 up and diluted the pigment, thus bringing it nearer the 

 surface ; but, from a microscoi)ical examination of the 

 feathers, the granules of pigment in the brighter ft;athers 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 careTully distinguished from th:? 

 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 at)raded 

 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 iiilected by their Structare," by 

 Dr. Hans Gadow (P. Z. S. 1882, ]x 409). 



