Our Bird Comrades

by Leander S. Keyser

*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?