It's about to become rush hour on the migration highway, as billions of birds gear up to leave the northern regions where they raise their young each year. They're headed for a six-month hiatus in places where food will be plentiful, such as the American South and the tropics.
Many kinds of birds -- shorebirds, raptors, water birds, songbirds -- lift off from breeding grounds as far north as the edge of the Arctic Circle, as far east as Nova Scotia and as far west as Alaska.
Some will fly during the day, while other species choose to travel at night.
Birds of prey, like osprey, bald eagles, red-tailed hawks and broad-winged hawks, are among the daytime flyers, employing a savvy strategy that conserves energy and maximizes distance traveled. They fly into rising columns of warm air -- thermals -- which act like elevators, whirling them high into the sky. At the top of the column they glide out, effortlessly making forward progress before joining a new thermal and repeating the process. This is a way for big, heavy birds to cover distances without having to do much tiring wing flapping.
On the other hand, most small songbirds fly at night and work for every mile they cross. Night's cooler weather keeps them from overheating as they flap their wings, and darkness keeps them from flying into a river of day-flying predatory birds. Night flight allows birds like orioles, warblers and thrushes to land and grab a quick nap as the sun comes up, then spend the day foraging to build up energy for the coming night's flight.
Shorter days
Everyone notices when the days start getting shorter. While this may make us humans lament the approaching end of summer, it causes migratory birds to galvanize into action. Shorter days get translated into hormone shifts inside bird bodies, leading to profound changes.
After the rigors of summer, birds' outer feathers need replacing, so before taking off on their long flights, they molt a new set. (These often are less eye-catching than their summer feathers, since males no longer need to engage in courtship.)
Hormonal changes tell birds to begin gorging on seeds, grains or fruit to quickly put on fat. Packing fat all over their bodies is like filling up the gas tank of the car -- it's fat that fuels migration. Many birds face long intercontinental journeys, with uncertain food resources along the way, so they need to carry some reserves.
How do birds know when to leave their summer homes? Hormones have been sending signals to their brains, making them increasingly restless as the days pass. Finally they feel compelled to lift off and head south, at about the same time each year, largely independent of weather conditions. If an oriole began its migration on Aug. 22 last year, that bird probably lifted off about the same time this year, barring storms or cloudy nights.
Commuter lanes
Although birds nested across the continent in a huge, broad swath this past summer, most will funnel down to one of four heavily traveled lanes, called migratory flyways, to continue their migration. In our region, many birds fly within the Mississippi Flyway, using the great river as a guide. This is especially true for large waterfowl, like Canada geese, and small songbirds, like warblers and thrushes.
How tiny beings weighing a few ounces can survive storms and loss of habitat, tall buildings and utility lines in their flight path, and cats and other predators when they land, is amazing. But most amazing of all is birds' ability to navigate along their long routes, and do it so precisely that they often land in or near the same tree they left last spring. It's not that they have a firm map embedded in their brains, either.
How do they do this? Scientists are learning more all the time about avian navigation. They've proved that birds use information from the setting sun and star patterns, from the Earth's magnetic fields and shifts in polarized light to orient themselves. Then they add in information from such landmarks as rivers and mountain ranges, and possibly even scents from the ground below, to guide them along.
About 85 percent of the birds that nested in our state this summer are now gearing up to leave us until next spring. You could go outside on a night this fall and hear migrants calling as they pass overhead, a stirring sound and one that clearly heralds the changing of the seasons.
Val Cunningham, a St. Paul nature writer, bird surveyor and field trip leader, can be reached at valwrites@comcast.net.
VIEW THE MIGRATION
There are many good spots for viewing day-flying migratory birds (or day-resting night flyers). These include:
Hawk Ridge in Duluth, especially for eagles, hawks and falcons.
Gooseberry Falls State Park northeast of Duluth, for many kinds of migrants.
Old Cedar Avenue Bridge area in Bloomington, for waterbirds, songbirds and raptors.
Lilydale and Crosby Regional Parks in St. Paul, for a varied cast of birds.
