The Descent of Man

A few words must be added on changes of plumage
in relation to the season of the year. From
reasons formerly assigned there can be little
doubt that the elegant plumes, long pendant
feathers, crests, etc., of egrets, herons, and many
other birds, which are developed and retained
only during the summer, serve for ornamental
and nuptial purposes, though common
to both sexes. The female is thus rendered more
conspicuous during the period of incubation
than during the winter; but such birds as
herons and egrets would be able to defend
themselves. As, however, plumes would probably
be inconvenient and certainly of no use
during the winter, it is possible that the habit of
moulting twice in the year may have been gradually
acquired through natural selection for
the sake of casting off inconvenient ornaments
during the winter. But this view cannot be extended
to the many waders, whose summer
and winter plumages differ very little in colour.
With defenceless species, in which both sexes,