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ready to mate often enough, is that one sex—the male—shall be more
constantly in the phase of sexual preparedness, and by his display
shall both advertise the fact and also help to stimulate the female to
the proper emotional level.
Finally, as we have mentioned, there is a more direct biological
advantage in display. It appears that in seasons which have been
inclement just before and during egg-laying, the number of eggs is
often reduced and the percentage of infertility raised. It is also
known that all the reproductive processes of birds are very much
under the control of the higher, emotional centres of the brain. For
instance, a female dove brought up in isolation from infancy will
usually lay no eggs; but the presence of a male bird in a near-by
cage, or even the caressing of her neck with a human finger in a way
reminiscent of the caresses of the male's nibbling beak, will almost
always cause an egg to be laid. It has now been demonstrated that
display and threat promote the ripening of the reproductive organs;
this will be of advantage, especially in bad seasons, since birds'
emotions arc very much at the mercy of the weather.
Before leaving this group, mention should be made of the curious
fact that in all-the-year residents who are also territory-birds, there
is an *' engagement" period in the spring. For some weeks after the
pair are in possession of a territory, fertilization is not effected. The
biological reason for this is plain—it is advantageous for a bird to be
on its territory early, or it may not find one; but it must not breed
before a date which will, give the probability of there being plenty of
food for the young. The physiological machinery by which it is
effected resides in the female; it is only at a certain season (probably
depending on a certain mean temperature) that the eggs in her ovary
start to grow rapidly, and only then that her full sex-instincts arise.
Finally, we come to the large group of birds in which both male
and female not only help look after the young, but also share in in-
cubation and in the building of the nest. Such are the herons, the
pelicans, the grebes, the divers, and many others. In them, neither
parent is biologically the more precious; so that if protective colour
is needed, it is needed by both. Furthermore, their instincts have to
be so similar in regard to nest, eggs, and young that the similarity, it
appears, has spread to their courtship habits, too. For it is at any
rate a fact that in a large number of this group of birds, and nowhere
else, we find what we must call mutual courtship—both sexes develop-
ing bright colours, and special structures for the breeding season, and
both using them simultaneously in a mutual display (which, as with
other monogamists among birds, begins only after pairing-up).