Blyth, in Ibis, 1867, p. 173.)
That aquatic birds have acquired a white plumage so much oftener than
terrestrial birds, probably depends on their large size and strong
powers of flight, so that they can easily defend themselves or escape
from birds of prey, to which moreover they are not much exposed.
Consequently, sexual selection has not here been interfered with or
guided for the sake of protection. No doubt with birds which roam over
the open ocean, the males and females could find each other much more
easily, when made conspicuous either by being perfectly white or
intensely black; so that these colours may possibly serve the same end
as the call-notes of many land-birds. (54. It may be noticed that with
vultures, which roam far and wide high in the air, like marine birds
over the ocean, three or four species are almost wholly or largely
white, and that many others are black. So that here again conspicuous
colours may possibly aid the sexes in finding each other during the
breeding-season.) A white or black bird when it discovers and flies
down to a carcase floating on the sea or cast up on the beach, will be
seen from a great distance, and will guide other birds of the same and
other species, to the prey; but as this would be a disadvantage to the
first finders, the individuals which were the whitest or blackest would
not thus procure more food than the less strongly coloured individuals.
Hence conspicuous colours cannot have been gradually acquired for this
purpose through natural selection.
As sexual selection depends on so fluctuating an element as taste, we
can understand how it is that, within the same group of birds having
nearly the same habits, there should exist white or nearly white, as
well as black, or nearly black species, for instance, both white and
black cockatoos, storks, ibises, swans, terns, and petrels. Piebald
birds likewise sometimes occur in the same groups together with black
and white species; for instance, the black-necked swan, certain terns,
and the common magpie. That a strong contrast in colour is agreeable to
birds, we may conclude by looking through any large collection, for the
sexes often differ from each other in the male having the pale parts of
a purer white, and the variously coloured dark parts of still darker
tints than the female.
It would even appear that mere novelty, or slight changes for the sake
of change, have sometimes acted on female birds as a charm, like
520