resemble each other, but in which the summer and winter plumage differ
slightly in colour. The difference, however, in these cases is so small
that it can hardly be an advantage to them; and it may, perhaps, be
attributed to the direct action of the different conditions to which
the birds are exposed during the two seasons. Thirdly, there are many
other birds the sexes of which are alike, but which are widely
different in their summer and winter plumage. Fourthly, there are birds
the sexes of which differ from each other in colour; but the females,
though moulting twice, retain the same colours throughout the year,
whilst the males undergo a change of colour, sometimes a great one, as
with certain bustards. Fifthly and lastly, there are birds the sexes of
which differ from each other in both their summer and winter plumage;
but the male undergoes a greater amount of change at each recurrent
season than the female of which the ruff (Machetes pugnax) offers a
good instance.
With respect to the cause or purpose of the differences in colour
between the summer and winter plumage, this may in some instances, as
with the ptarmigan (79. The brown mottled summer plumage of the
ptarmigan is of as much importance to it, as a protection, as the white
winter plumage; for in Scandinavia during the spring, when the snow has
disappeared, this bird is known to suffer greatly from birds of prey,
before it has acquired its summer dress: see Wilhelm von Wright, in
Lloyd, Game Birds of Sweden, 1867, p. 125.), serve during both
seasons as a protection. When the difference between the two plumages
is slight it may perhaps be attributed, as already remarked, to the
direct action of the conditions of life. But with many birds there can
hardly be a doubt that the summer plumage is ornamental, even when both
sexes are alike. We may conclude that this is the case with many
herons, egrets, etc., for they acquire their beautiful plumes only
during the breeding-season. Moreover, such plumes, top-knots, etc.,
though possessed by both sexes, are occasionally a little more
developed in the male than in the female; and they resemble the plumes
and ornaments possessed by the males alone of other birds. It is also
known that confinement, by affecting the reproductive system of male
birds, frequently checks the development of their secondary sexual
characters, but has no immediate influence on any other characters; and
I am informed by Mr. Bartlett that eight or nine specimens of the Knot
(Tringa canutus) retained their unadorned winter plumage in the
Zoological Gardens throughout the year, from which fact we may infer
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