Birds. 3975 



beyond them for the true cause. Now I have said above, that consti- 

 tutional weakness seems to me to be the real root of the matter. I 

 conceive that on a strong and healthy bird, none of the supposed 

 causes above named would operate to the variation of its colour ; but 

 that on birds of peculiar weakness, these or any other external acci- 

 dents or derangements of their natural habits do sometimes induce 

 such unwonted hues, which perhaps are still oftener the result of de- 

 bility alone, without any other particular promoting cause. Mr. Yar- 

 rell seems to be of this opinion; for in speaking of the common linnet 

 (Brit. Birds, i. 554), he says, " males do not, in confinement, acquire 

 the fine red colour which pervades the breast of a mature wild bird;" 

 "the particular plumage assumed during the breeding season by many 

 species, being a periodical indication of constitutional and sexual 

 vigour." And again, in vol. ii. p. 101, in speaking of a young rook of 

 a light ash-colour, mottled over with black, and with the quill and 

 tail feathers elegantly barred, he says that upon the bird moulting, all 

 its mottled plumage vanished entirely ; and also adds, " this agrees 

 with my observations : accidental varieties will generally be found to 

 be smaller and weaker birds than those which are truly characteristic 

 of the species ; as these young birds increase in age, and gain consti- 

 tutional powers, the secretions become perfect, and the plumage as- 

 sumes its natural colours : the assumption of white feathers by old 

 birds is probably the effect of the converse operation of this physio- 

 logical law." 



Presuming then that physical weakness is the radical origin of the 

 varieties in colour so often seen, whether they are developed with or 

 without any farther exciting cause, I come now to enumerate those 

 instances of varieties in colour which I have been able to gather, with 

 some few particulars regarding them ; and I have been at some pains 

 to collect all the instances of varieties for which I could find good 

 authority, because I conceive, that if resulting from weakness, such de- 

 formities must occasionally occur in every species of bird : and I have 

 little doubt, that if we had equal opportunities of observing them, we 

 should find albino and mottled varieties among our rarer birds, no 

 less than among the thrushes, blackbirds, sparrows, robins and rooks, 

 which come so frequently under our notice. 



Golden Eagle. " White varieties have been seen and recorded," 



Yarrell. 

 Peregrine Falcon. White varieties were so frequent as to cause, 

 among naturalists, the introduction of a new specific name for 

 them, under the impression that they were a distinct species 



