time is a most extraordinary circumstance, that seems to bid defiance
to all human reasoning. But the believer in the gradual modification
of species will be far from feeling surprise at finding gradations of
all kinds. If the male pin-tail were to acquire his new plumage within
a still shorter period, the new male feathers would almost necessarily
be mingled with the old, and both with some proper to the female; and
this apparently is the case with the male of a not distantly-allied
bird, namely the Merganser serrator, for the males are said to undergo
a change of plumage, which assimilates them in some measure to the
female. By a little further acceleration in the process, the double
moult would be completely lost. (83. See Macgillivray, Hist. British
Birds (vol. v. pp. 34, 70, and 223), on the moulting of the Anatidae,
with quotations from Waterton and Montagu. Also Yarrell, History of
British Birds, vol. iii. p. 243.)
Some male birds, as before stated, become more brightly coloured in the
spring, not by a vernal moult, but either by an actual change of colour
in the feathers, or by their obscurely-coloured deciduary margins being
shed. Changes of colour thus caused may last for a longer or shorter
time. In the Pelecanus onocrotalus a beautiful rosy tint, with
lemon-coloured marks on the breast, overspreads the whole plumage in
the spring; but these tints, as Mr. Sclater states, do not last long,
disappearing generally in about six weeks or two months after they have
been attained. Certain finches shed the margins of their feathers in
the spring, and then become brighter coloured, while other finches
undergo no such change. Thus the Fringilla tristis of the United States
(as well as many other American species) exhibits its bright colours
only when the winter is past, whilst our goldfinch, which exactly
represents this bird in habits, and our siskin, which represents it
still more closely in structure, undergo no such annual change. But a
difference of this kind in the plumage of allied species is not
surprising, for with the common linnet, which belongs to the same
family, the crimson forehead and breast are displayed only during the
summer in England, whilst in Madeira these colours are retained
throughout the year. (84. On the pelican, see Sclater, in Proc. Zool.
Soc. 1868, p. 265. On the American finches, see Audubon,
Ornithological Biography, vol. i. pp. 174, 221, and Jerdon, Birds of
India, vol. ii. p. 383. On the Fringilla cannabina of Madeira, Mr. E.
Vernon Harcourt, Ibis, vol. v. 1863, p. 230.)
416