142 TlMEHRl. 
among the Macoosi Indians, where the growth of yellow, 
instead of red, blue, or green, feathers is obtained in par- 
rots and macaws, the change in colour is evidently- 
brought about, as suggested by Mr. IM Thurn, by the 
change of food — a change through which certain essen- 
tial constituents of the red, blue and green pigments are 
not elaborated, probably because they are absent from the 
food eaten in captivity. Chemical analyses of the original 
red, blue and green pigments and of the resulting yellow 
pigment, are much to be desired in this case ; for then we 
should know definitely not only what are the essential con- 
stituents of the red, blue, and green colours, and of the 
resulting yellow tint ; but also the reason why the change 
in each case is to one uniform yellow colour. Possibly 
some metallic constituent is present in the original pig- 
ments, as in the African Plantain-eaters ; and this me- 
tallic constituent may be different in the different colours 
or may consist of one element differently combined. It 
would not then be difficult to account for the fa6l that, 
the metallic element or elements not being elaborated 
because they are absent from the food eaten in captivity, 
a uniform, yellow, basal pigment is produced. 
A change, produced by artificial means, in the colora- 
tion of birds, and one to which I can find no reference 
in any published volume, seems to be more or less com- 
monly pratlised by bird-stuffers in the colony — I refer to 
the change of the natural purple tints in the colours of the 
Cotingas, the purple being changed to a lively red by the 
application of heat, in each case, to the feathers of the dead 
bird. It seems possible that this change can also be pro- 
duced in the feathers of the living bird ; but I have not been 
able to obtain any satisfactory information on this point. 
