XIII 
THE COLOURS OF BIRDS 
273 
colour, with a tendency to exhibit a longitudinal 
edging of olive colour in which the barbules are 
pale-coloured, unconnected, and slightly modified. 
This pale band is sometimes replaced in the greater 
wing-coverts of the male by a dark brown band ; in 
the tail-quills of the male usually by a metallic 
band which, in the case of the central rectrices, may 
invade the whole vane. The development of lipo- 
chrome colour or of transverse bands of colour does 
not occur in the case of quill-feathers. The latter 
is due to the fact that in sun-birds it is only the 
barbules which stand near the distal end of the barb 
which tend to become metallic, the result being the 
formation of transverse stripes of bright colouring 
on short feathers, and longitudinal stripes on long 
feathers, the type developed having a definite rela¬ 
tion to the length of the feather (see Fig. 1). This 
peculiarity is apparently the result of the fact that 
in metallic barbules the lamina tends to disappear, 
and this seems to occur typically only in downy 
barbules or in the barbules standing near the apices of 
the barbs. Downy barbules never become metallic, 
so that it is only the apical barbules which can 
become metallic, and give rise to a band of colour. 
From the above description it is obvious that the 
development of brilliant colouring in sun-birds is 
certainly associated with modifications of feather 
structure which cause the feathers to deviate more 
or less completely from the primitive type of feather 
structure. 
T 
