XIII 
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. 1). 
Relation between Structure and Colour 
We have already seen that there are three great 
sets of colour phenomena displayed by the feathers 
of birds:—(1) 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 
