The virtuosos will not arrive until mid-April. But the second-string players check in from late February on. Some of them never left the outdoor symphony hall, but have been dully silent since autumn. As the grass greens and the sun grows stronger, songbirds tune up for the stunning performances that will begin in mid-spring.
In a Zone 5 climate, the robins, not true migrants, begin returning from their winter foraging grounds in early March, but they are not in full voice quite yet. Nor are the goldfinches or bluebirds that hunkered down through winter’s snows. Yet March is when all these songbirds begin some serious vocalizing.
There is a clear difference from one month earlier, when the snow-muffled silence was broken only rarely by any semblance of avian melody. In an East Coast winter, the melody is kept alive by only a handful of species – like the cheerful Carolina wren, energetically repeating his three-note refrain, or the great horned owl solemnly broadcasting his five notes through the wooded darkness. Otherwise, the birds of winter are taciturn, giving voice mainly to their alarm at predators or their annoyance with competitors for food and shelter.
By March, however, the utterances take on more complexity and frequency. Although ornithologists caution against anthropomorphic inferences, it is hard to resist the notion that singing birds are rejoicing over the balmier weather and more abundant food supplies.
What makes a songbird a songbird? The short answer is a highly developed, well-muscled syrinx, the voice-box. It is situated low on the windpipe, much closer to the lungs than the human larynx is to human lungs. Not all birds have a syrinx. The turkey vulture lacks one, for example, and thus can do little more than hiss.
Songbirds account for the overwhelming majority of bird species. But just as a working larynx does not guarantee that a woman will be a coloratura soprano, not all songbirds are adept vocalists just because they are equipped with a syrinx. The syrinx’s complexity varies from species to species. This is why a tiny hooded warbler can out-sing a mighty crow – technically a fellow songbird – any day of the week.
But even the most proficient songbirds need proper incentive to do their best work. That incentive will come soon enough, in the form of compelling reproductive urges. The male gives his best musical performances to attract a mate and to ward off rivals for that mate’s attention. The vigor of his song advertises his genetic suitability to procreate – and his ability to defend territory and thus the future of the brood he and his mate will produce.
With due apologies to science, it is hard to reduce all birdsong to cold zoological facts. To human ears, the ebullient refrain of a robin on a sunny April day equates with sheer joy. The liquid, achingly lyrical song of the wood thrush sounds like nothing less than a celebration of life itself.