xm 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. i). 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. 



