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