The Descent of Man

also, finds that all male birds with rich or
strongly-characterised plumage are more quarrelsome
than the dull- coloured species belonging
to the same groups. The goldfinch, for
instance, is far more pugnacious than the linnet,
and the blackbird than the thrush. Those
birds which undergo a seasonal change of plumage
likewise become much more pugnacious
at the period when they are most gaily ornamented.
No doubt the males of some obscurelycoloured
birds fight desperately together, but it
appears that when sexual selection has been
highly influential, and has given bright colours
to the males of any species, it has also very often
given a strong tendency to pugnacity. We
shall meet with nearly analogous cases when
we treat of mammals. On the other hand, with
birds the power of song and brilliant colours
have rarely been both acquired by the males of
the same species; but in this case the advantage
gained would have been the same, namely success
in charming the female. Nevertheless it