3o8 A HISTORY OF BIRDS 



— we have, in conclusion, a reversal of the usual law of plumage 

 evolution. For herein the males are practically indistinguish- 

 able and the females quite distinct ! A study of nestlings may 

 give us the clue to this mystery. 



Finally, mention must be made here of the phenomena of 

 albinism and melanism, about which but little is yet known. 



In nine cases out of ten the recorded instances of the occur- 

 rences of albino birds turn out to be merely "white varieties, as 

 is shown by the fact that the iris and the coverings of the beak 

 and feet retain their normal colouring. While among some 

 species white varieties are common, as for instance among 

 Blackbirds, in others such variations are extremely rare. 



As to the causes which produce either white varieties or 

 true albinos nothing is known ; but a few curious facts have 

 been collected which are worth recording. 



White varieties of black or grey plumaged birds are rela- 

 tively common, while they appear excessively rare among birds 

 of green plumage. Between absolutely white and albino 

 varieties and the normal plumage almost every gradation in 

 shade may be met with, and this seems to show that the lack 

 of colour is due to weakness in the secretion of pigment. Isa- 

 belline varieties, for example, are the natural consequence of 

 the " watering-down,'' so to speak, of melanin pigments and are 

 not infrequently met with in birds such as Rooks and Starlings, 

 for example. 



The Hon. Walter Rothschild has pointed out that most if 

 not all green birds show yellow varieties, and this is but the 

 natural consequence of the " watering-down " of the pigment just 

 referred to, though at the same time it must be remarked 

 that this green colour is not due to green pigment, but to dark 

 melanin pigments and structural interference. White varieties 

 of the Grey Parrot {Psittacus erithacus) have been several times 

 recorded, but generally the red tail is retained ; a specimen, 

 however, in Mr. Rothschild's collection has a white tail with 

 the normal grey plumage of the rest of the body. Red appears 

 to be less easily changed. White Bullfinches, for example, re- 

 tain the red of the under-surface. 



Very frequently the assumption of white feathers is only 

 partial, hence "pied" birds, where white feathers are mixed 

 with normal plumage. 



