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 booklets. 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 orsome 

 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 



