Pick your favorite sign of spring: squirrels mating, mud oozing, maples flowering. Mine is a vulture soaring.
Change in the air is a naked, ruddy head gliding in on big wings. But more than being a vernal messenger, the turkey vulture is an avian iconoclast. It topples simplistic notions of migration.
We can usually predict, within days, the return of many species from far away, such as Bicknell’s thrush (from the Caribbean), warbling vireo (from Mexico and Central America), or bobolink (from South America). These are complete migrants; all of them left us last fall, and they return, predictably, each spring.
But a number of species migrate on the cheap. These partial migrants fly south for winter, but not too far south. Among certain species of partial migrants, some members head south while others move short distances or even remain in the breeding region.
The eastern bluebird is an example. One of America’s most elegant songbirds is a mess when it comes to distribution and migration. Consider its range map. To be sure, most bluebirds left the northern portions of the breeding range last fall. Yet a few hardy individuals regularly pass the winter in places as cold as Vermont or north of the Great Lakes. The same goes for American robins, red-winged blackbirds, and other “harbingers of spring.” Overall, yes, these species migrate. But among them we find mavericks in the cold, particularly during winters with abundant food supplies.
Migration isn’t necessarily an easy option. Falling temperatures and dwindling food resources do drive birds southward in fall, but migrating to avoid winter’s hardship is itself risky. Beyond the caloric demands of flying south (and then north again in spring), stopover sites along the way might hold unreliable food or unfamiliar predators. Some migrants lose prime wintering sites altogether – to storms, for example, or commercial development. Imagine yourself on a plane for Brazil, arriving to find that your airport has vanished.
But there’s another reason to stay put. It’s a guy thing. Among some partial or short-distance migrants, males linger farther north than females. In winter or early spring, for example, you will probably find more males among groups of American robins or dark-eyed juncos. In some species, the relative abundance of females increases steadily southward.
A classic explanation for the overabundance of males in winter is that turf matters. By migrating short distances, or by never leaving, northern-residing males get first dibs on the better territory. So, counterbalancing the risk of death in the cold is the benefit of breeding success in spring. And the winners’ offspring might themselves inherit the fitness to stay north next winter.
It’s a convenient hypothesis, natural selection in action. But is it true? Beware of easy answers. Perhaps males are more abundant in the north simply because they out-compete females for scarce food resources. In the struggle for existence, males will indeed be aggressive toward females over food. Fatter, larger males fare better in the cold.
It’s likely that all these explanations fit in one way or another, which brings me to the turkey vulture, a partial migrant whose northernmost members retreat south in the fall. I expect to find robins, bluebirds, and juncos here in Vermont each winter, but rare are turkey vultures in the cold. They winter in southern New England and points south.
But around the middle of February, as the sun drags itself higher above the horizon each day, before red-winged blackbirds sing “honk-a-ree,” before American woodcocks begin their frenetic courtship flights, and before eastern phoebes arrive in the yard, a turkey vulture drifts north on teetering wings and a balmy breeze. It isn’t spring. Not yet. But the vulture, searching for something dead, is a reliable emissary for a season’s rebirth.
Bryan Pfeiffer is an author, wildlife photographer, guide, and consulting naturalist who specializes in birds and insects. He lives in Montpelier, Vermont.