Songbirds' chorus

Listening to my friend sing, I am moved by the diverse vocalizations of her song. More than the words, each note and tone carries a message, a mood, a limitless range of communication.

As with her song, I am taken with the song of the birds. Each morning I wake to the dawn symphony. Like the vocals of my friend's song, I am captivated by the subtle nuances of the birds' chorus. The more I listen and believe I am learning their song, the more I begin to see the intricacy and depth of their communication. Just when I think I know a bird's call, I watch the same bird croon a new, throaty song, completely different from the one I had heard. What does the new message convey?

Rather than dismay, I anticipate each morning's chorus with yearning. I am inspired by the song of the day and filled with questions. Which bird will sound first? What inspires this bird to wake before the others? Who are the latecomers into the chorus? Did they have a late night? And most intriguing, what are they singing about?

Danger, food, shelter

Beyond their well-evolved eyesight, birds are dependent on their sense of hearing for survival, using that sense to detect prey. A barn owl can hunt mice by sound alone; woodpeckers can hear insects moving under the bark of a tree; pigeons can detect the infrasonic waves (under 16 vibrations per second) that come just prior to an earthquake.

But it is the auditory stimuli of other birds that is often most important. Auditory signals help birds detect and locate danger, territory, food and shelter. Birds hear a greater range of sounds than humans. But when looking at individual bird species, the tendency is toward a more specialized narrow range.

Air supply

Allowing this range of vocalizations is the syrinx, an organ unique to birds. The syrinx is located just below the trachea and above the fork of the bronchial tubes. In the most basic way, the syrinx is related to the mammal larynx in that it produces sound when air is forced through the trachea, causing membranes to vibrate. But unlike the larynx, the syrinx is much more efficient, utilizing nearly all the air that passes through to create sound. In comparison, the human larynx only uses 2 percent of the air exhaled from the lungs.

Beyond this efficiency, the syrinx has adaptations which allow a phenomenal range of sound. The syrinx sits upon two tracheal tubes leading to the bronchial tubes. Each tracheal tube is equipped with a separate sound-producing membrane and individual neural connections.

Ordered to sing

The dual membranes and neural connections allow birds to create two sounds at once. Because of this adaptation, birds can sing from one side of the syrinx, taking quick breaths during a pause.

The intricacy and ability of birds to sing has been associated with the use of the syringeal muscles. Birds such as storks and vultures have very limited vocalizations and have been found to be unable to use the musculature of the syrinx. The complexity of this musculature has been used taxonomically to create two suborders, oscines and suboscines, in the Passeriformes order.

Passeriformes are known as the passerines, or songbirds. Oscine is derived from the Latin canere, "to sing." Therefore, amongst the passerines the oscine suborder has complex syringeal musculature and extensive song capability. The suboscines are more similar to birds outside the passerine order. Another way these suborders differ is the way in which they come to their song. Oscines must learn their songs; the suboscines' song is innate.

Feathers funnel sound

Coupled with extensive vocalizations is a bird's sense of hearing. Although not readily visible, birds do have ear canals that are protected by specialized feathers called auriculars or ear coverts. These feathers do little to interfere with sound but help to funnel sound into the ear passage to the cochlea, the sound receptive organ.

Owls are an excellent example of how facial and head feathers help direct sound. Differing from mammals, though, birds have more simply designed ears, with only one bone to transmit sounds rather than three, but birds' hearing does not seem to be lessened.

While we have extensive knowledge of how birds listen to and create songs, huge gaps remain in our understanding of their communication. Maybe the only way to decipher the language of the birds is to truly listen.

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