The Descent of Man

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 possible serve the same end as the callnotes
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