BIRDS--KINGS OF THE AIR
A bird is an animal clothed with feathers and having the forelimbs
adapted to flight.
The birds constitute a class in the phylum Chordata, and otherwise are
combined, in the group Sauropsida, with the Reptilia, with which they
agree more closely in anatomy than with any other group, one prominent
particular being that both have a single condyle, in contrast with the
mammals and amphibians where the condyle is double. In fact primitive
reptiles--probably of the stock of dinosaurs--are the ancestors of
birds, the divergence having occurred probably in Carboniferous time.
Of the earliest divergent forms, the rocks have as yet yielded no
specimens, the most ancient bird forms recovered showing a degree of
development in the new type that must have been preceded by a long
history of evolution from its reptilian source.
[Illustration: ARCHÆOPTERYX
Skeleton of Archæopteryx macrura with indication of feathers
(Reconstructed. After Andrea)]
The oldest fossil bird known is that named archæopteryx, whose remains
are found in the Jurassic slates of Bavaria, which represent the
beginning of the Mesozoic or Age of Reptiles. In much of its anatomy,
and in the possession of perfect feathers, it is a true bird, yet it
retains many reptilian features. Its body was about the size of a small
crow; its legs were rather long, with well-developed feet of four
toes suitable to grasping a perch; its wings were short and probably
feeble, for the shoulder girdle and ribs are weak and the sternum is
rudimentary. It is plain that it was arboreal in habits, but a poor
flyer, and was aided in scrambling about the branches of trees on whose
leaves and bark it may have fed, by the fact that three digits of the
rather lizardlike wing hand terminated in strong claws, while the
thumb was entirely free.
The practical value of this clawed hand is illustrated in a living
bird--the hoatzin, of northern South America--which exhibits in several
ways the probable appearance and manners of the archæopteryx. "It
haunts the sides of lagoons and rivers where a thick growth of low
trees projects over the stream or the mud left bare by the tide. When
disturbed the bird flies off awkwardly with a violent flapping motion,
or leaps from bough to bough, erecting its crest and expanding its
wings and tail. The note is sharp and shrill, and has been described
as a hissing screech. The food consists of leaves and fruit. The
conspicuous nest, placed on low trees or shrubs, is a loose platform of
spiny sticks and twigs with a softer lining, and contains from three to
five yellowish eggs, spotted with reddish brown and lilac. The young,
which can see and run as soon as they are hatched, have a claw on both
forefinger and thumb, by means of which they creep about the thickets,
and hook themselves over the branches, assisted by the bill and feet.
They can also swim and dive."
The most striking features of the archæopteryx were its head and
tail. The skull is fairly avine, and the rather short and blunt bill
was furnished with conical teeth, nearly equal in size, and set in a
marginal row in distinct sockets. Still more lizardlike was the tail--a
prolongation of the backbone nearly as long as the body, along each
side of which sprouted strong feathers forming a horizontally flat tail
with a rounded end.
[Illustration: TOOTHED BIRD
(_Hesperornis regalis_)
Skeleton of toothed bird (After Marsh)]
The next that we know of bird evolution is derived from the discovery
of the fossil remains of toothed birds in the Upper Cretaceous
formations of Kansas--that is, in the more recent half of the Mesozoic
Age. They differ greatly not only from archæopteryx but from each
other, and are represented by several species. One type (Hesperornis)
was a wingless, diving bird of great size, whose long, heronlike
beak was studded with small, sharp teeth, all alike, implanted in a
continuous groove; its legs were so hinged to the compressed pelvis
that they could be extended almost level with the back, and the lobed
toes thus became lateral winglike paddles of great power. The other
type, represented by Ichthyornis and its relatives, also had a long,
stout bill set with teeth, but each in a separate socket. Ichthyornis
was about the size of a pigeon, and its strongly developed wing bones
and deeply keeled sternum show that it was a bird of powerful flight,
and apparently gull-like habits. So far as we know neither of these
Cretaceous birds had any progeny. When, after an immensely long period,
other fossils come to light in rocks of the middle Tertiary period they
bear few traces of ancestry, and exhibit little relation to the great
mass of modern orders. They are the "flightless birds," possessing no
wings but running about on massive legs; and the group includes the
extinct æpyornis, dinornis, and moa, and the existing ostriches, rheas,
emus, cassowaries, and kiwis. Some ornithologists question whether this
"ratite" group, characterized by having no "keel" on the sternum, did
not have an origin and line of descent quite distinct from those of
both the Cretaceous toothed birds and the modern "carinate" type which
possess a medial crest or "keel" on the breastbone for the support of
the flight muscles; but the more general opinion is that they are a
variant from very early birds with wings.
HOW A BIRD IS BUILT
Since its feathers are the one thing that marks a bird, outwardly,
as different from other classes of animals, we ought first of all to
learn what feathers are, and what purpose they serve. A quill feather,
such as may be picked up in any farmyard, has a horny, hollow stem
or "shaft," which is bare at the closed large end or "base," but has
two soft, winglike expansions toward its tapering end that together
make its "vane." This thin, flat vane consists of delicate branches,
"barbs," studded with tiny hooks, the "barbules," holding each adjacent
branchlet in place, but letting the whole vane bend and spring. The
whole beautiful thing is really very strong and elastic, as it must be
to push as hard against the air as a bird's wing has to do. The vanes
vary much in shape, and in the degree to which the branchlets are
disconnected into a fluffy looseness. Ostrich plumes, and those of the
birds of paradise, owe their beauty to the fact that each branch in the
vane is loose, and bears little disconnected branches of its own; and
in many feathers no vane at all grows, so that they resemble hairs,
when fine, and bristles when coarse, as is seen about the mouth of the
whippoorwill and some flycatchers. The nestling plumage or "down" is
of this character. The lovely plumes of egrets are slender stems of
feathers having in place of a vane scattered soft hairs. In some sea
birds the feathers are so stiff and hard as to be almost like scales.
Those of water birds, and especially the divers, are wonderfully close,
thick, and greasy, so that the down that forms an undercoat for warmth,
and the skin beneath it, never gets wet.
Feathers, then, serve their wearers first of all as clothing--very
thick and warm in birds of cold places; and doubtless this beneficial
modification of the primitive reptilian scale, by reason of its
conserving the warmth of the body, and gradually increasing the
temperature of the blood, has been largely instrumental in enabling
birds to rise so far above the grade of their cold-blooded and sluggish
ancestors.
Most animals whose lives are spent in the open air and light show more
or less color in their coat, but none are more beautifully adorned than
birds. The most brilliant examples are to be found in the tropics, and
some of the gayest in our colder land, such as the tanagers and humming
birds, are strays from large tropical families noted for gaudy attire.
The color we see in plumage may be due to either of two conditions.
It may, as is usually the case, be simply coloring matter deposited
in the substance of the feathers. But where the plumage gleams with
changing rainbow lights, as on the fiery throat patch of the humming
bird, on the neck of a dove or on the purple-black coat of the grackle
(crow blackbird), these splendid reflections are caused by very minute
wrinkles on the feathers, that break up the light. It is the same
effect, called "iridescence," as is seen on the mother-of-pearl and on
a soap bubble. Blue is usually an effect produced by certain coloring
matter not blue underlying a thin covering of feather substance; and
when you pound a blue feather into dust that dust will be black or
gray--or, at any rate, not blue. Birds of the same group are colored
much alike, as a rule.
In some cases the style of colors worn appears to be the best for the
safety of the birds of the group by making them hard to see as long
as they keep still. Thus most birds whose lives are passed on or near
the ground, and which build their nests there, are dull in coloring;
they are in danger from more enemies than are tree-dwelling birds, and
must be able to hide better. No bird of nocturnal habits is brightly
colored. It is mostly among the small, quick-flying species, such as
warblers and finches, that we find the gayly dressed ones. They are
birds of the sunshine, and usually migratory. In most cases when birds
have a plain dress there is little difference in it between the male
and the female; but whenever you find a species of bird wearing a gay,
ornamental dress, it is almost always the male that sports these fine
feathers, while the female and young are clothed in dull yellow, drab
or brownish tints. This appears to be another measure of safety. The
males can wander about, look out for themselves, and take to flight
when danger threatens; but their mates must sit quietly on their nests,
and trust for safety for themselves and (what is really more important)
their eggs or young mainly to not being seen. In their plain colors
they blend into the foliage and the shadows amid which they sit, and so
are more likely to escape the sight of prowling foes.
Feathers are not intended to remain permanently; they become worn and
faded, or are lost, so that at regular intervals the bird needs a new
suit of clothes. Twice a year, therefore, in spring and autumn, they
are pushed out by new ones sprouting in the same feather-growing pits.
This shedding of the feathers is called "molting," and it is analogous
to the shedding of the outer, horny pellicle of its skin by a snake or
lizard. Their molting is not very noticeable in land birds, because
the feathers drop out little by little; otherwise the poor creatures
would be left quite naked, and unable to fly. In most birds the new
feathers that come in are the same in pattern and color as those they
displace, so that the new plumage differs little if any from season to
season; but some birds acquire a new coat for winter that is decidedly
different, and sometimes snowy white, making them inconspicuous amid
the snow.
The largest and most important feathers in a bird's outfit are those
of the wings and tail, by means of which it flies and controls its
progress. How birds are able to keep themselves aloft in the air,
and move through it at will, is not yet understood. That it requires
great strength of wing muscles, and rigid support for them is evident.
Therefore we find the head of the arm bone (humerus) fastened by stout
ligaments to a great shoulder blade sunk in the flesh beside the fore
part of the spine, and also braced in two directions by other interior
bones, one of which extends down to join its opposite fellow at the
front end of the breast bone, and form the "wishbone" (the united
coracoids). This solid bracing by bones and tying by ligaments gives
the needed firmness to the wings; and enables their powerful muscles to
work them.
How great these muscles are you will know when I tell you that the
thick mass of "white meat" in the breast of the fowl carved at your
table consists only of the two principal muscles that move the wings
when a downward stroke is made. They, in their turn, are attached at
the base to the broad surface of the breastbone, or "sternum" and its
projecting keel. Beyond the wrist joint stretches a large, misshapen
hand, which consists mostly of one great forefinger, in the tough flesh
of which the big quills, or outer flight feathers, called "primaries,"
are rooted. Lying over their bases, when the wing is folded, is a row
of somewhat smaller quill feathers called "secondaries." Above those
are the small and close "wing coverts."
The tail is very important in guiding and checking a bird in flight,
and is useful in various other ways, and may also be extremely
ornamental. The tail quills are always in pairs, making an even number
of feathers. This results from the reduction to a mere stub of the
long clumsy tail worn by the archæopteryx and its fellows. The quills
continued to grow in pairs out of the side of the tail as it diminished
until all that there is room for (ten or twelve pairs) are now rooted
side by side around the edge of the condensed coccygeal bones.
Birds are, as a class, the most active of animals, and their
temperature is highest; this means a large consumption of oxygen, and
the windpipe is usually capacious, yet the lungs are not large, but
are supplemented by another apparatus for aeration. Opening out of the
lungs are several pairs of air sacs, amplest in those birds that are
much on the wing, which not only occupy spaces between the muscles and
organs within the chest, but in many cases extend into the neck and
head, and even into the limb bones, which in most birds are hollow.
[Illustration: THE PELICAN, NOTABLE FOR ITS THROAT POUCH
Photo, Keystone View Co.]
Here is a suitable place to say a few words about how a bird sings. The
breath enters and leaves the windpipe through the larynx in the back of
the mouth--an organ which, in our throats, contains the vocal cords and
voice-producing apparatus; but in birds the larynx is unimportant, for
their voice organ is near the lower end of the windpipe, and is called
"syrinx" or music box. It consists of an enlargement and modification
of the bony rings about the windpipe at the point where it forks into
the two branches to the lungs; and incloses vibrating membranes. It is
also furnished with small muscles that act to expand or contract the
tube and its inner fixtures, thus regulating the column of air forced
through the syrinx when the bird calls or sings. These muscles thus
control the space and the shape of the opening, and the tension of
the membranes that serve as vocal cords. The muscles of the syrinx vary
greatly in number and efficiency among birds; and many kinds classed as
"singing birds" (Oscines) do not sing melodiously or tunefully because
their music box is imperfectly supplied with the proper muscles. They
have the instrument, but are unable to play upon it.
[Illustration: PEACOCK WITH BRILLIANT TAIL SPREAD
Photo, Keystone View Co.]
[Illustration: SACRED PHEASANT
Photo, A. N. Mirzaoff]
* * * * *
The 10,000 or more different kinds of birds now living in the world are
classified in fifteen orders, of which the lowest in rank is that of
the ostriches, and allied ratite birds, mostly extinct, that stand in a
place apart by reason of their archaic structure and inability to fly.
The ostrich is still wild in the arid districts of Africa, Arabia, and
Mesopotamia; the rhea is Patagonian; the emus and cassowaries belong
to Australia and New Guinea; the apteryx, or kiwi, still survives in
New Zealand; and several gigantic ratite birds have recently become
extinct in New Zealand and in Madagascar, where egg shells, laid by the
prehistoric æpyornis, that will hold two gallons are still found. Some
species of these birds were seven feet in height.