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Wetmore, Alexander, 1886-, et al. / Warm-blooded vertebrates
(1931)

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


Page 20


BIRDS
Unlike chicks and ducklings, whose bodies are com-
pletely covered with down when they emerge from the
egg, the young of many of our common birds (like the
warblers, sparrows, flycatchers, and thrushes, which re-
main helpless in their nests for some time after hatching)
have only little tufts of hairlike down scattered scantily
over the head and body. Such tufts are wholly insuffi-
cient to aid in keeping them warm. A few, like the blue
jay, loggerhead shrike, frigate bird, and pelican, are
naked when first hatched from the egg but develop com-
plete or partial coverings of down later. The nakedness
of these young when born into the world represents an
extreme in specialization.
Though superficially the body of the adult bird seems
uniformly covered with feathers, closer examination re-
veals that feathers grow in definite tracts of the skin and
that the intervening spaces are bare (Fig. 3). In young
chickens in which the growth of the body has outstripped
the growth of its feather covering, the feather tracts and
bare spaces show clearly. The arrangement of the feather
tracts in most birds follows the same general pattern:
The head has a rather even covering of feathers; a tract
runs down the back, enlarging in its central section;
another follows each side of the breast; and the wings and
the upper parts of the legs, as well as the tail, have each
its own tract. Many specialized birds exhibit variations
from this general pattern, the bare head of birds like the
turkey buzzard exhibiting one of the most obvious.
Feathers grow from a little point sunk in a tubelike pit
known as the feather follicle. From this point an elon-
gated papillus develops and pushes rapidly out as a pin-
feather, which soon bursts its sheath to spread out in the
familiar structure of the feather. When fully developed,
growth ceases and the feather receives no further nour-
ishment from the body of the bird. It is then a dead
structure, like the free end of a finger nail, and may be cut
without sensation on the part of the bird. The feather is
1! 2o]


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