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Wetmore, Alexander, 1886-, et al. / Warm-blooded vertebrates

Chapter II: adaptations for progression by flying ,   pp. 13-24 ff. PDF (4.7 MB)

Page 19

mum size, the powder-down feather continues to grow
as long as the bird lives. The powder produced is not oily
as is usually supposed, its smooth feeling when rubbed
between the fingers being due to the microscopic size of the
flexible plates of horny material that compose it.
The use of these curious powder downs has aroused con-
siderable discussion. Many have believed that in herons
at least they are luminous and attract fishes at night, but
there is no basis for such a belief. The truth is that the
powder downs are used to dress the feathers, in the same
manner as is the oily secretion produced by the oil gland
at the: base of the tail. I have observed in young captive
herons that the powder downs developed simultaneously
with other body feathers, while the oil gland did not be-
come functional in producing its oily fluid until the bird
had reached a later stage of growth. When young herons
kept in captivity preened their feathers they rubbed the
bill repeatedly in the extensive powder-down patches on
either side of the breast until they had gathered a small
amount of the powder on the tips of the mandibles, and
then they pulled body and wing feathers quickly through
the bill to distribute this substance over the surface of the
quills, where it evidently served as a dressing.
In some parrots powder downs occur singly, scattered at
random over the body. These birds distribute the powder
among the plumes by fluffing the feathers and then shak-
ing them quickly. At times, with such quick movement,
small amounts of the powdery substance rise in the air
about the bird like fine, barely perceptible dust.
A striking peculiarity of powder downs lies in the fact
that though found in only a small number of species, these
are scattered through the bird kingdom apparently at
random. For example, powder downs occur in all the
herons, in the marsh hawk and many other hawks, and
in some parrots, but in the great group of perching birds,
only the wood swallows (Artamidae), found in Australia
and the Orient, have them.

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