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stimulating effect than uniformity. So, granted a tendency to marked
variation, variety will be encouraged and preserved.
This clear-cut case is of importance, because it enables us to draw
pretty definite conclusions in other similar cases. In the blackcock,
for instance, a handsome member of the grouse tribe, there are similar
assembly-places for mating—veritable temples of Venus. Here the
individual males cannot be distinguished, but each again appears to
have his own definite pitch or stand, and, both from direct watching
and by analogy with the ruff, it seems that here, too, there is true
selection. Finally, in some birds of paradise there are also mating-
placcs, but in the trees, where the males dance and display their
gorgeous plumes.
It is interesting to note that the evolution of such special mating-
places with assemblies of males and visits by females has taken place
at least three separate times in birds—in the waders, the game-birds,
and the birds of paradise. The influence of mode of life on type of
courtship is another problem that can be followed out in birds.
Where there is polygamy and where the female alone broods the eggs
and cares for the young, there we find the greatest disparity in colour
and courtship-behaviour between the sexes. The female is generally
drab, protectively coloured; the male, per contra, brilliant, and alone
participating in display. Since there is polygamy (or promiscuity),
the successful male will imprint his characters on a larger number of
descendants—and so display-brilliance will be at a premium; while,
since he plays no biologically useful role after fertilization is once
effected, there is less need for protective colour, since it does not much
matter whether he be killed or no.
Most birds are monogamous, however, at least for the season (or
sometimes only for a single brood—like the American wren, which as
bird-banding experiments have shown, usually changes partners
between the first and second broods of a single year). Most of the
largest group of monogamous birds, the song-birds proper, have their
whole sex-life hinge on what we may call the territorial system. They
have their young hatched naked and helpless, needing abundant food
for their growth, and liable to die of cold if left too long unbrooded.
Hence it is necessary, first, for both parent birds to feed the young;
second, for the presence round the nest of an area sufficiently large to
supply the young's needs, and not trespassed upon by other food-
seeking parents of the same species. This is ensured through an
extension of the instinct, nearly universal among birds, to resent
intrusion into the area round the actual or future nest-site.
Even in colonial nesters, like egrets or guillemots, the defended area