The Descent of Man

That male birds should sing from emulation as
well as for charming the female, is not at all
incompatible; and it might have been expected
that these two habits would have concurred,
like those of display and pugnacity. Some authors,
however, argue that the song of the male
cannot serve to charm the female, because the
females of some few species, such as of the canary,
robin, lark, and bullfinch, especially when
in a state of widowhood, as Bechstein remarks,
pour forth fairly melodious strains. In some of
these cases the habit of singing may be in part
attributed to the females having been highly
fed and confined (32. D. Barrington, 'Philosophical
Transactions,' 1773, p. 262. Bechstein,
'Stubenvogel,' 1840, s. 4.), for this disturbs all
the functions connected with the reproduction
of the species. Many instances have already
been given of the partial transference of secondary
masculine characters to the female, so that
it is not at all surprising that the females of some
species should possess the power of song. It