Feathers 55 
are almost always due to pigment or colouring-matter in 
the shaft or vane. If we take a black feather and hold 
it to the light, it will still look black; if we pound it with 
a hammer, it will not change. 
Green is never found as a pigment except in the 
feathers of a small family of birds called plantain-eaters 
or turacous, which inhabit West Africa. For some time 
it was thought that the natives dyed the birds artificially, 
as when these birds were kept captive, the magnificent 
scarlet patch on the wing would gradualky fade and 
become a dull gray. It is a fact that this colouring- 
matter washes out when the feather is washed in alkaline 
water. Even ordinary water will be slightly tinged if the 
feather is soaked in it. The pigment contains about ten 
per cent of copper, and this can be extracted chemically 
in the form of a metallic powder. The plumage of almost 
all brightly coloured birds will fade in the course of years, 
if the feathers are left exposed to direct sunlight; but, like 
photographic plates, the hues of some birds are more sen- 
sitive than others to the light. The delicate reds and 
yellows on the lower parts of Mexican Trogons are par- 
ticularly evanescent, and the rose-pink of the African 
Fairy Warbler disappears a short time after death. 
We might speak of a third class of colours, which are 
due to both pigment and structure. For instance, no 
blue pigment is known to exist in the feathers of birds, 
but blue feathers contain a brown or yellowish pigment 
which is encased in the horny coating of the feather. 
Between this outer sheath and the underlying pigment 
is a layer of many-sided cones or small projections which 
