278 
COLOUR IN NATURE 
CHAP. 
find that metallic colours in birds are of the type 
described in sun-birds, i.e. are due to the conversion 
of the distal portion of the terminal barbules into a 
club-shaped body consisting of a series of overlap¬ 
ping compartments, the process being accompanied 
by the total suppression of cilia and hooklets. Such 
a modification of structure is apparently of very 
common occurrence in birds, but does not give rise 
to metallic colour unless there is a simultaneous 
development of a large amount of black pigment. 
Colours of the Birds of Paradise 
We may now for a little pass to the consideration 
of the birds of Paradise, which on account of their 
greater size afford more obvious illustrations of some 
colour problems. The birds of Paradise as a group 
exhibit an extraordinarily specialised type of colora¬ 
tion, the specialisation being visible alike in colour 
and in structure. Though probably nearly allied to 
the crows, the development of melanin pigment is 
here less remarkable than the display of bright pig¬ 
mental colours. These are in part due to lipochromes, 
but in part, as we have already seen, to the pigment 
zoorubin, which is almost confined to the group. 
Further, we have not only the display of tufts and 
crests of additional feathers, as in humming-birds, 
but we find that these feathers are modified in every 
conceivable way, sometimes being reduced to mere 
wires, and at other times displaying brilliant metallic 
colours. Among many of the birds of Paradise the 
metallic colours are of somewhat limited distribution, 
contrasting with the pigmental colours rather than 
