Summer weather has hit with a vengeance. Starting last Thursday, we were awoken by thunderstorms three mornings in a row, and by yesterday, when the dawn chorus could finally return to normal, the wood thrushes were much subdued. I suspect that they have paired up and are getting down to the serious and urgent business of building nests and laying eggs.
Be that as it may, I did get a bit of a consolation prize yesterday when I went for a dawn walk down along Laurel Ridge. I’d been startled by the sight of the rising sun glowing a lurid red through the mountain laurel. Then when I dropped down a little off the ridgetop to follow an old woods road we call Ladyslipper Trail, I was even more surprised to hear a Swainson’s thrush calling – for the second time in five days. The first time I heard him, he was on the ridgetop right up from the house, and we naturally assumed that he was just passing though. The fact that he is still here raises the possibility that he may be thinking about setting up shop, well south of the normal range for his species. This happened three years ago, when I heard a Swainson’s singing for about three weeks on another part of the property. I got the impression that that one hadn’t been very successful in attracting a mate.
The song of the Swainson’s thrush unmistakably belongs to the same group as wood thrush, hermit thrush and veery. It has the same bell-like quality, resembling a hoarser version of the hermit thrush – a very quiet series of ascending phrases. As my brother Mark once pointed out, for the thrush family, generally speaking, the relative loudness of the call indicates the type of habitat preferred. The loudest songs belong to the denizens of hedgerow and dooryard, such as the American robin, while the quietest are the deep woods or high altitude specialists, who have to compete only with the soughing of wind in conifers. The Swainson’s call is very much in this latter category.
Last week, when I was looking through the book Bird Sounds, by Barry MacKaye, I was struck by his description of the mechanics of avian vocalization. Instead of a voicebox, birds have a unique structure called the syrinx, located right where the trachea and the windpipe divide. The position and musculature are designed to take maximal advantage of the fact that birds resemble nothing so much as flying bellows. In addition to the lungs proper, they have nine air sacs distributed throughout the chest and abdomen, some of which may even extend into the bones in some species. The main evolutionary “purpose” of these structures, of course, is to provide bouyancy and improve gas exchange. Any given breath, MacKaye says,
may be stored for more than one cycle of inhalation and exhalation. Initially, the puff of air does not go directly into the lungs, but passes through them without gas exchange. The puff of air goes directly to the rear part of the bird’s body. First, most of the air the bird inhales reaches the posterior air sacs, including the large, paired abdominal air sacs. Parts of the lungs also receive air, and there is a subsequent exchange of gases as the breath passes through the lungs on the way to the back of the bird. But as the bird breathes out, the posterior air sacs contract, pushing the puff of air into the lungs, completing the first cycle of the bird’s two-step breathing process.
As the bird breathes in again, while the new air enters the bird via the route just described, the initial puff exits most of the lungs and enters the anterior air sacs via lung bronchi, where more gas exchange occurs. This single breath of air carries with it some of the warmth generated by the bird’s metabolic processes . . . The body contracts, and the initial puff of air leaves via the anterior air sacs at the front of the bird and the bronchial tubes that lead up from the lungs past the point where they join the trachea, and out. Put simply, the same breath of air passes through the lungs twice, although not the same parts of the lungs.
Thus, whereas humans, for example, “use only about 2 percent of the air column that passes out of the respiratory system in making vocalizations . . . songbirds use nearly 100 percent of the air column to produce song.” And the songbird’s complex anatomy allows the harmonic blending of different tones and even, in the case of some species such as the brown thrasher, two wholly separate songs, sung simultaneously.
The bird’s control of the configuration of the syrinx and associated sound-producing anatomy is so finely tuned that it can operate one side of the syrinx independently of the other. By rapidly altering the configuration of the trachea, throat and mouth, the bird can focus the two separate elements into the single complete song. Like a pianist’s two hands playing tune and harmony, a bird can blend two separate sounds into a pleasing harmonic.
Pretty nifty, eh? I can’t help recalling the lyrics from an old “concept album” called 2112, from the Canadian hard rock band, Rush —
We are the priests of the temples of Syrinx.
All the gifts of life are held within our walls.
— which leads in turn to hazy memories of bowls and water pipes and puffs of air being breathed and re-breathed, strained by one pair of lungs and then another, capturing every last trace of blue until our bodies filled with light and the music on the stereo slowly turned itself inside-out. Ah, if only paranoia hadn’t forced us always to lock ourselves in, the dawn choruses we could’ve heard . . . the lives that could’ve been saved, I think, through such revelations . . .