You know what it’s like: the alarm goes off, it’s dark, the wind’s howling, your nose is cold, there’s a snow drift on the windowsill, the radio is talking about icy streets and flesh freezing in 30 seconds, and you just want to pack up and get out of town.
Yes, I know I wrote almost that identical paragraph to open my recent column on hibernation, but this is the age of reusing and recycling, and this week I’m writing about another way to escape winter: migration.
Unlike hibernation, migration is something humans are capable of and undertake in large numbers every winter. But when they get to their place in the sub-tropical sun, they may find they’ve been preceded by the most famous migrators of them all: birds.
Not that birds are the only migrators. Millions of wildebeest follow a great circular route around the Serengeti grasslands every year in the wake of the summer rains. Canadian caribou range up into the Arctic tundra during the summer, then travel more than 800 kilometres south as snow covers the grazing. Atlantic green turtles migrate more than 2,000 kilometres from their breeding grounds on the mid-Atlantic Ascension Island to the Brazilian coast. And monarch butterflies that hatch in the Great Lakes region fly 3,000 kilometres south to winter in Florida and Mexico, then head north in the spring. Most die on the way, but they hatch offspring that finish the journey for them.
Migration occurs because any population of animals remaining in one place too long will eventually exhaust the food supply, especially if that supply fluctuates with the seasons. Birds are the most famous migrators because they have wings: their flight makes them more visible and allows them to migrate great distances.
We’re very familiar with migratory birds on the Great Plains because it’s one of the great flyways of the world. Most of the geese, ducks, swans and other birds (including the rare whooping crane) that fly over our heads follow a north-south pattern: they have breeding grounds in the high northern latitudes and spend their winters in equatorial regions. (However, the champion migrator, the Arctic tern, takes north-south migration to something of an extreme, spending summers in the Arctic, then flying 6,000 kilometres to winter in the Antarctic — where, of course, it’s summer again.)
Even an experienced human pilot doesn’t undertake a flight of 6,000 kilometres lightly. How these bird-brains do it is still not fully understood. It appears, though, that they use one of, or a combination of, two processes: piloting and orienteering.
Piloting means flying from one easily remembered landmark to the next, following the Mississippi River, for instance, or looking for conspicuous mountains or lakes. Since this kind of information can’t be passed on genetically, birds that use piloting have to learn the route from older birds. Canada geese, for example, accompany their parents on their first migration, and in the middle of the following year make a practice run over the route before accompanying their parents again for the real migration in the autumn.
Some birds, though, fly long distances over open water: the slender-billed shearwater follows a figure-eight migration path around the Pacific that takes it over 32,000 kilometres of open sea That requires orienteering: taking cues from the environment to ensure they’re headed the right way. Prevailing winds and characteristic cloud formations that form over ocean currents might provide clues, and some nocturnal migrants, like human sailors, guide themselves by the Pole Star. Pigeons use the position of the sun, and can orient themselves to the Earth’s magnetic field (on cloudy days, pigeons wearing strong magnets to confuse their sense of that field fly off in random directions).
Piloting and orienteering, while impressive, aren’t the same as true navigation, which requires a map at which the navigator looks to calculate his route from point A to point B. Various experiments to demonstrate that homing pigeons have internal maps, and are therefore true navigators, have been inconclusive.
Whether it’s based on internal maps or not, the long-distance migration of birds is one of the most spectacular animal achievements. It’s an achievement we honor by emulation, calling those of us who fly south for the winter “Snowbirds.”
Real birds, however, don’t have to wait for seat sales.
