two white spots on the primaries more conspicuous than in the female.
(25. For the Milvago, see Zoology of the Voyage of the Beagle,
Birds, 1841, p. 16. For the Climacteris and night-jar (Eurostopodus),
see Gould s Handbook to the Birds of Australia, vol. i. pp. 602 and
97. The New Zealand shieldrake (Tadorna variegata) offers a quite
anomalous case; the head of the female is pure white, and her back is
redder than that of the male; the head of the male is of a rich dark
bronzed colour, and his back is clothed with finely pencilled
slate-coloured feathers, so that altogether he may be considered as the
more beautiful of the two. He is larger and more pugnacious than the
female, and does not sit on the eggs. So that in all these respects
this species comes under our first class of cases; but Mr. Sclater
( Proceedings of the Zoological Society, 1866, p. 150) was much
surprised to observe that the young of both sexes, when about three
months old, resembled in their dark heads and necks the adult males,
instead of the adult females; so that it would appear in this case that
the females have been modified, whilst the males and the young have
retained a former state of plumage.)
We thus see that the cases in which female birds are more conspicuously
coloured than the males, with the young in their immature plumage
resembling the adult males instead of the adult females, as in the
previous class, are not numerous, though they are distributed in
various Orders. The amount of difference, also, between the sexes is
incomparably less than that which frequently occurs in the last class;
so that the cause of the difference, whatever it may have been, has
here acted on the females either less energetically or less
persistently than on the males in the last class. Mr. Wallace believes
that the males have had their colours rendered less conspicuous for the
sake of protection during the period of incubation; but the difference
between the sexes in hardly any of the foregoing cases appears
sufficiently great for this view to be safely accepted. In some of the
cases, the brighter tints of the female are almost confined to the
lower surface, and the males, if thus coloured, would not have been
exposed to danger whilst sitting on the eggs. It should also be borne
in mind that the males are not only in a slight degree less
conspicuously coloured than the females, but are smaller and weaker.
They have, moreover, not only acquired the maternal instinct of
incubation, but are less pugnacious and vociferous than the females,
and in one instance have simpler vocal organs. Thus an almost complete
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