any bright
colour at all, and rank no higher than our thrushes in ornamental
plumage. Of the humming-birds the same may be said, except that the
females are often green, and sometimes slightly metallic, but from their
small size and uniform tints are never conspicuous. The glorious blues
and purples, the pure whites and intense crimsons of the male chatterers
are represented in the females by olive-greens or dull browns, as are
the infinitely varied tints of the male tanagers. And in pheasants, the
splendour of plumage which characterises the males is entirely absent in
the females, which, though often ornamental, have always comparatively
sober and protective tints. The same thing occurs with many other
groups. In the Eastern tropics are many brilliant birds belonging to the
families of the warblers, flycatchers, shrikes, etc., but the female is
always much less brilliant than the male and often quite dull coloured.

_Cause of Dull Colours of Female Birds._

The reason of this phenomenon is not difficult to find, if we consider
the essential conditions of a bird's existence, and the most important
function it has to fulfil. In order that the species may be continued,
young birds must be produced, and the female birds have to sit
assiduously on their eggs. While doing this they are exposed to
observation and attack by the numerous devourers of eggs and birds, and
it is of vital importance that they should be protectively coloured in
all those parts of the body which are exposed during incubation. To
secure this end all the bright colours and showy ornaments which
decorate the male have not been acquired by the female, who often
remains clothed in the sober hues which were probably once common to the
whole order to which she belongs. The different amounts of colour
acquired by the females have no doubt depended on peculiarities of
habits and of environment, and on the powers of defence or of
concealment possessed by the species. Mr. Darwin has taught us that
natural selection cannot produce absolute, but only relative perfection;
and as a protective colour is only one out of many means by which the
female birds are able to provide for the safety of their young, those
which are best endowed in other respects will have been allowed to
acquire more colour than those with whom the struggle for existence is
more severe.

_Relation of Sex Colour to Nesting Habits._

This principle is strikingly illustrated by the exist