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Gender Dimorphic Bills
Gender Dimorphism is mostly restricted to peculiarly
shaped bills; for instance, the horn of the male Hornbills is often larger,
and differs in shape from that of the female. In the males of Pelicans
several unpaired excrescences are formed entirely by the horny coating
of the premaxilla; they sometimes reach a height of three to four inches,
and are again cast off after the breeding season, resembling in the latter
feature the Auks.
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The most striking example of dimorphic bills is
that of the New-Zealand HUIA, Heterolocha, the bill of the female being
slender, about four inches long, and much curved, while that of the male
is nearly straight, stout, and scarcely half that length. The knobs or
swellings in the Gallinae are mostly restricted to the males; the same
applies to Oedemia (SCOTER). Sexual differences in colour are common. For
instance, in the male Scoter the bill is black and orange, in the young
and in the female it is simply grey, and without the knob. The bill of
the adult male Blackbird is orange-yellow; that of the young of both sexes
and of the adult males of Buceros malayanus
(HORNBILL) is white,
but becomes black in the adult female, forming thus an interesting exception
to the general rule that the young agree with the females, and that aberrant
coloration is confined to the males. The colour of the bill is deposited
as a diffused pigment in the horny cells of the epidermal coat, but is
occasionally restricted to the deeper layers, or even to the Malpighian
layer itself, then shining through the outer transparent layers.
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The Egg Tooth
In connection with the bill is to be mentioned the
"egg-tooth," which is developed in the embryos of all birds as a small
whitish protuberance or conglomeration of salts of calcareous matter, deposited
in the middle layers of the epidermis of the tip of the upper bill, without
being connected with the premaxilla itself. The sharp point of this "tooth"
soon perforates the upper layers of the horny sheath, and then files through
the eggshell, a slight crack in the latter being sufficient to enable the
young bird to free itself. A similar egg-tooth exists in Reptiles, and
is, as in Birds, cast off after hatching. The wearing away of the growing
and constantly renewed .horny layers of the bill can be easily observed
in the pealing beak of a Parrot.
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