Migration’s Misfits

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: 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. The Cliff Swallows returning on schedule to San Juan Capistrano? To tell you the truth — no big deal.

But a number of species migrate on the cheap. These partial migrants fly south for winter, but not too far south. And certain partial migrants are variable: some of them head south while others move only short distances or even remain in the breeding region.

The Eastern Bluebird is an example. One of America’s most elegant songbirds is a lovely mess when it comes to winter distribution and migration. Consider its range map (courtesy of the Cornell Lab of Ornithology’s exquisite All About Birds site). 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.

American Robin

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 Dark-eyed Juncos or American Robins (among the reasons the robin is a badass bird). 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 dibs on the better territory. So, counterbalancing that 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 back 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 here 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 usually drifts north on teetering wings and a balmy breeze. (Here’s a map of Turkey Vulture sightings since February 1. [you’ll need an eBird account to see it].)

No, it isn’t spring. Sorry, not yet. But the vulture, searching for something dead, is nevertheless a reliable emissary for a season’s rebirth.