For me, one of the great joys of music is beautiful harmony. How I would love to be able to sing along with friends and harmonize, but my singing abilities are suspect at best. So, I sit back and enjoy the talents of others.
We each get one voice. Some singers are sopranos, others a gravelly bass. But good or bad, high pitch or low, harmonizing with oneself is out of the question. It takes two or more singers to create this kind of magic.
Not so with birds. A great many have two voices, and some do harmonize with themselves. A little review of anatomy is in order.
Our voice comes from our larynx. This organ, which is nearly impossible to spell correctly, sits atop the trachea in the throat, and the vocal cords (folds of tissue actually) vibrate with the passage of air creating sounds and speech or, at times, beautiful music.
Birds don’t have a larynx. Instead, they have a syrinx — just as difficult to spell correctly. It sits farther down in the chest where the trachea branches into the two bronchi, one to each lung. If you can recall a little high school biology, the trachea and bronchi are semi-rigid tubes that are supported by rings of cartilage. They are held together by flexible membranes.
The syrinx of a bird has a series of tiny muscles attached to the trachea and bronchi (more in some species than others) that can pull and tug on the cartilage rings either tightening the membranes or allowing them to go slack. When tightened they vibrate with the passage of air producing sound. The tighter the membranes the higher the pitch. In addition, there are flaps of tissue that, when allowed to press together, vibrate when air is forced between them.
It gets really interesting when you realize that much of this sound-producing apparatus is divided between the two bronchi. This gives birds the potential to sing with two voices at the same time. They can duet with themselves and, yes, even harmonize.
Our thrushes excel at this. If you look at a spectrogram of a hermit thrush song, it is amazing. A spectrogram is a picture of a song and reads like sheet music. The higher the pitch of the note, the higher the note is located on the spectrogram.
The spectrogram of the thrush reveals a long, pure tone falling slightly in pitch followed by a jumble of sweet flute-like notes, many overlapping, as the sounds from the two sides of the syrinx combine to produce a rich medley of music with harmony. This is probably why a great many birders consider the hermit thrush to be the best vocalist in North America.
I’ve never heard a nightingale sing, but it, too, is a thrush, and its song is just as complex.
Our Swainson’s thrush is equally talented. The abundant notes overlap as the song spirals upward. To a lesser degree, Townsend’s solitaires and even the robin in your backyard demonstrate the same skills. And this ability is not limited to thrushes. On the other hand, you will never hear sweet harmony from a vulture or pelican. They lack a syrinx entirely.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.