The Descent of Man

or the males alone, become extremely conspicuous
during the breeding-season,—or when
the males acquire at this season such long wing
or tail-feathers as to impede their flight, as with
Cosmetornis and Vidua,—it certainly at first
appears highly probable that the second moult
has been gained for the special purpose of
throwing off these ornaments. We must, however,
remember that many birds, such as some
of the Birds of Paradise, the Argus pheasant
and peacock, do not cast their plumes during
the winter; and it can hardly be maintained that
the constitution of these birds, at least of the
Gallinaceae, renders a double moult impossible,
for the ptarmigan moults thrice in the year.
(33. See Gould's 'Birds of Great Britain.') Hence
it must be considered as doubtful whether the
many species which moult their ornamental
plumes or lose their bright colours during the
winter, have acquired this habit on account of
the inconvenience or danger which they would
otherwise have suffered.