In ducks the sexual characters of the male differ from those in the fowl,
especially in the fact that they almost completely disappear after the
breeding season and reappear in the following season. In the interval the
drake passes into a condition of plumage in which he resembles the female;
and this condition is known as 'eclipse.' The male plumage, therefore, in
the drake has a history somewhat similar to that of the antlers in deer.
Two investigations of the effects of castration on ducks and drakes have
been recorded. H. D. Goodale [Footnote: 'Castration of Drakes.' _Biol.
Bulletin_, Wood's Hole, Mass., vol. xx., 1910] removed the generative
organs from both drakes and ducks of the Rouen breed, which is strongly
dimorphic in plumage. One drake was castrated in the early spring of 1909
when a little less than a year old. This bird did not assume the summer
plumage in 1909, that is, did not pass into eclipse. It was in the nuptial
plumage when castrated. This breeding or nuptial plumage is well known: it
includes a white neck-ring, brilliant green feathers on the head, much
claret on the breast, brilliant metallic blue on the wing, and two or more
upward curled feathers on the tail. The drake mentioned above was
accidentally killed in the spring of 1910. Another drake was castrated on
August 8, 1909: only the left testis was removed, the other being
ligatured. At this time the bird would be in eclipse plumage. It appears
from the description that it assumed the nuptial plumage in the winter of
1909, and did not pass into eclipse again in the summer of 1910. Thus in
drakes the effect of castration is that the secondary sexual character
remains permanently instead of being lost and renewed annually. Goodale,
however, does not describe the moults in detail. In the natural condition
the drake must moult twice in the year, once when he sheds the nuptial
plumage, and again when he drops the summer dress. Goodale insists, from
some idea about secondary sexual characters which is not very obvious,
that the eclipse or summer plumage is not the same as that of the female.