xm THE COLOURS OF BIRDS 265 



region. Barbs bearing barbules of this type are 

 the only ones present in down feathers ; these are 

 further characterised by the shortness of their axis, 

 and are rarely important in coloration, except in the 

 young. 



The small feathers which cover the surface of the 

 body differ in several respects from quills. They 

 are much shorter, the quill region is practically 

 absent, the rachis is reduced in length and thickness, 

 and the downy region tends to be more fully de- 

 veloped. The result of this shortening of the axis 

 is that the barbs tend to radiate from a common 

 point, while in feathers with elongated axis they are, 

 roughly speaking, parallel to one another. This has 

 an important bearing upon the coloration, for it is 

 obvious that if the barbules standing near the ends 

 of the barbs tend to exhibit special colours, then 

 the colour will form a transverse band on short 

 feathers, a longitudinal band on long feathers. 

 Similarly the median barbules will form a trans- 

 verse band on short feathers, a V-shaped marking 

 on long feathers ; both of these actually occur (see 

 Fig. I). 



Relation between Structure and Colour 



We have already seen that there are three great 

 sets of colour phenomena displayed by the feathers 

 of birds : — ( i ) The feathers may show beautiful 

 and complex markings in brown, gray, and black ; 

 (2) they may display vivid optical colours ; or 

 finally (3) may contain brightly coloured pigments, 

 usually of the nature of liprochromes. Of these 



