The generation of the egg after copulation and the generation of
the chick from the subsequent hatching of the egg are not brought
about within equal periods for all birds, but differ as to time
according to the size of the parent-birds. The egg of the common hen
after copulation sets and matures in ten days a general rule; the
egg of the pigeon in a somewhat lesser period. Pigeons have the
faculty of holding back the egg at the very moment of parturition;
if a hen pigeon be put about by any one, for instance if it be
disturbed on its nest, or have a feather plucked out, or sustain any
other annoyance or disturbance, then even though she had made up her
mind to lay she can keep the egg back in abeyance. A singular
phenomenon is observed in pigeons with regard to pairing: that is,
they kiss one another just when the male is on the point of mounting
the female, and without this preliminary the male would decline to
perform his function. With the older males the preliminary kiss is
only given to begin with, and subsequently sequently he mounts without
previously kissing; with younger males the preliminary is never
omitted. Another singularity in these birds is that the hens tread one
another when a cock is not forthcoming, after kissing one another just
as takes place in the normal pairing. Though they do not impregnate
one another they lay more eggs under these than under ordinary
circumstances; no chicks, however, result therefrom, but all such eggs
are wind-eggs.