*Reprinted by permission from "Our Animal Friends."
Not a little discussion has arisen among the dissectors as to the
anatomy of bird song. Into this controversy I shall not enter--at
least, not in a controversial spirit--but shall recount only what may
be regarded as the best and latest results of scientific research. How
does a bird produce the melodious notes that emanate from his throat?
Are they manufactured far down in the trachea, or only at its anterior
opening? Are they voice tones or flute tones? These questions will be
answered as we proceed to examine the bird's lyrical apparatus without
going into wearisome detail, or making use of many difficult scientific
terms, which are the bane of the general reader.
Let me begin at the upper end of the avian singing machine--that is,
with the mouth, including the bill, the lips of which are called
mandibles. Just as the movements of the human lips have much to do
with the modifications of the human voice, so the opening and closing
of the bird's mandibles exercise a modifying influence upon avicular
tones. If it were not so, the feathered minstrel would not keep his
mandibles in such constant motion during his lyrical recitals. You
will notice that whenever he desires to strike a very high and loud
note he opens his mandibles quite widely, sometimes almost to the
fullest possible extent.
However, the expansion and contraction of the throat orifice, no doubt,
produce still more marked variations in the tones of the vocalist; yet
it must be borne in mind that closed or partly closed mandibles will
obstruct the passage of the air from the throat, while open mandibles
will permit of a full passage of the air current, and the tones will
vary accordingly. Besides, the roof of the bird's mouth is grooved or
convex, and therefore the character of the sounds will be somewhat
dependent upon the position and movement of the upper mandible.
And then there is the bird's tongue, which is constantly in motion
while the musical rehearsal is going on. Throughout its entire length
it can be raised and lowered at the bird's will, or be made to quiver
and roll, and by this means the air column forced up from the lungs is
manipulated in a wonderful way, producing in some cases an almost
unlimited variety of modulation.
Within the bird's neck two elastic tubes run down from the mouth into
the chest. One of them is the gullet or aesophagus, which is the
channel through which the bird's food descends into the crop and
gizzard. The other little cylinder lies in front of the gullet, and is
called the windpipe or trachea, and reaches down to the lungs, which
are the bellows furnishing the wind for the avian pipe organ. As Dr.
Coues says, the trachea is "composed of a series of very numerous
gristly or bony rings connected together by an elastic membrane," and
is supplied with an intricate set of muscles by which it can be
shortened or elongated at the will of the songster himself.
Now let us look at the upper end of this wonderful pneumatic pipe,
which so often throws Pan and all his coterie into a transport when the
thrasher and the wood thrush flute their dithyrambs. Here we find the
larynx. It is simply the anterior specialized portion of the trachea,
located at the base of the tongue, and in mammals is honored as the
voice organ, whereas in birds it is distinguished as the fluting
apparatus, the instrument that really produces the varied vocalization
of the bird realm. But the music is not the product of vocal cords, as
is the case in the human larynx, for at the upper end of the avian
larynx there is a slit or fissure, somewhat elliptical in form, and set
in the fork of the hyoid bone, which constitutes the bifurcated root of
the tongue. This fissure is called the glottis. At the bird's fiat,
it can be opened and closed and made to assume a great variety of
forms. Moreover, just in front of it there is a fold of mucous
membrane called the epiglottis, which is in reality a tiny trapdoor
closing over the opening when necessity requires. When the bird
swallows food or drink, this little flap shuts down, and prevents the
entrance of any clogging substance into the windpipe to choke the
feathered diner.
We have now come to the most strategic point in our investigation of
the anatomy of bird song, for in the avian world a special distinction
has been conferred upon that little orifice in the bird's throat called
the glottis. It is here that all the music, as well as all other
so-called vocal sounds, are generated--they are simply piped or fluted
through a slit, so that birds are _whistlers_, not singers or
vocalists. I repeat, so that my meaning may be perfectly clear--bird
music is not produced by means of vocal cords, as is the music of the
human throat, but by means of a whistling aperture in the larynx. And
that wonderful cleft has been placed there for that specific purpose.
Properly speaking, therefore, the feathered choralist does not have a
voice, but only a wind instrument; albeit a marvelous contrivance it is.
It will be easy now to see how the bird's tones are capable of a large
variety of modulations. The glottis is controlled by a system of
muscles that are perfectly obedient, within their limits, to the bird's
volitions, and thus it may be made to assume a great number of
different forms, each giving expression to a different vocal effect.
The shape of the glottis is also modified in numerous ways by the
movement of the tongue and mandibles. Nor is that all, for the air
column pumped up from the lungs may be increased or diminished at will,
a very strong current producing a loud tone, and a feeble current a low
one. The elongation or contraction of the whole throat will also
modify the pneumatic column, and thereby alter the quality of the tones.
We may go still further in our analysis. Suppose a bird should open
his mouth and throat as widely as possible, hold all his lyrical organs
steady, and blow his windpipe with all the strength his lungs could
command, it is obvious that the effect would be a clear, loud, uniform
whistle, such as the meadowlark sends across the green fields. But
suppose he desires to "blow a dreamy hautbois note, slender and refined
as ever stirred the air of Arcady or trembled in the vineyards of old
Provence," then all the musician in plumes needs to do is to contract
the slit in his throat, depress his tongue, almost close his mandibles,
and simply allow a slender air current to sift from the lungs through
the syrinx and out of the glottis. What if the whim should seize him
to pipe a trill or a quaver to the water witches of the meadow, as
Master Song Sparrow so often chooses to do? Then he simply needs to
set his tongue and throat to quivering, and you have his enrapturing
tremolo. Beautiful, is it not?
There are birds that send a kind of guttural sound from their throats,
such as the cuckoos and occasionally the blue jays. Notice the cuckoo
as he utters his call, which every swain interprets as the harbinger of
a coming shower, and you will observe that his throat bulges out like
that of a croaking frog, and quivers at the same time in a convulsed
way. It is plain that the air about to be forced from the glottis is
flung back by some muscular action and set to vibrating in the
laryngean cavity, thus giving the sound its croaking quality when the
elastic current is finally released.
Now, if the reader will pucker up his lips and whistle a tune, he will
notice that the sound is actually produced at the small labial orifice
and nowhere else; however, the tones are modified and modulated at will
in a variety of ways--by a deft, though almost imperceptible,
manipulation of the tongue, by a slight enlargement or contraction of
the aperture, and especially by a dexterous control of the air column
blown from the lungs. Just so the lyrists of fields and woods pipe
their roundels and _chansons_ through the chink in their throats, save
that in the bird's case the mouth and tongue are anterior to the
whistling aperture. I know a young man who has trained himself so as
to be able to mimic to perfection the complex songs of the western
meadowlark and the cardinal grosbeak. He does it by whistling.
Near the lower end of the trachea, just above the lungs, there is a
specialized organ of the bird's throat called the syrinx. It is a
cylinder formed of bony rings, provided with a mesh of muscles, and
having membranous folds which act as valves upon the two orifices of
the _bronchi_ leading to the lungs. Many scientific gentlemen have
declared that the syrinx is the voice organ of the birds, the elastic
margins of the folds or valves being set to vibrating by the projection
of the air from the lungs, and thus producing the varied lays we hear
in the outdoor concert. However, Mr. Maurice Thompson--who, by the
way, found time to do something else besides writing "Alice of Old
Vincennes," and something just as creditable to his talent,
too--dissected many birds with special reference to this subject, and
gave close attention to birds in the act of singing, both out of doors
and in captivity, and I am convinced that he proved the theory of the
syringeal origin of bird song to be an erroneous one.
Only two reasons need be adduced for this conclusion. First, it is
unreasonable to suppose that the rich, loud, clear notes of the
thrasher, the cardinal, and the mockingbird, lilting across the fields
and capable of being heard a long distance, are generated far down in
the lyrist's chest by the vibrating of the margin of a tiny mucous
membrane. If it had its genesis there, it surely would display a
muffled or guttural or sepulchral quality. In the second place, it has
been proved by actual dissection that the shrike, which possesses no
song gift worthy of the name, has a well-developed syrinx, while the
mockingbird, our feathered minstrel _par excellence_, has a syrinx that
is absolutely insignificant. On the other hand, the shrike's larynx,
including the glottis, is a clumsy affair, whereas the mocker's larynx
is indeed wonderfully made.
[Illustration: Meadow Lark]
It must not be supposed, however, that the syrinx does not perform an
important function in the production of avian melody. It acts as a
regulator or meter of the air impelled from the lungs. By means of the
folds or membranous valves the mouths of the bronchial tubes may be
opened widely or almost closed, and in this way, to quote from Mr.
Thompson, "the bird is enabled to measure in the nicest manner the
amount of air thrown from the lungs into the trachea." In producing a
staccato, for example, the valves flop up and down, doling out the air
at the proper intervals and in precisely the right quantities.
Indeed, nothing in the world of Nature is more wonderful than the gift
of bird song, and nothing proves more clearly the doctrine of design,
or, at least, of adaptation to a specialized purpose.