Photo: Tundra Swans, Randy Streufert
Tom L. Blackburn, Immediate Past President
The Tundra Swans have returned to the Great Marsh at Elizabeth Hartwell Mason Neck National Wildlife Refuge after a daunting migration of as much as 3,750 miles. Tundra Swans that winter in northern Virginia spend most of the year near the northern coasts of Canada and Alaska where they breed and raise their young. The young birds accompany their parents for their first fall migration, learning the route from older birds.
Tundra Swans, Randy Streufert
The ”why” of migration, particularly for far-north breeders like Tundra Swans, isn’t difficult to answer. Birds that don’t migrate in the fall are doomed to starvation as the shallow water, which they depend on for their main food supply of underwater vegetation, freezes over.
The “how” of migration has been much more difficult to figure out. More than half of Tundra Swans winter within 6 miles of the place they wintered the previous year, and some migrating birds use the same perch and nest boxes year after year. Scientists are still learning about the various methods birds use to get from “there” to “here” with such precision.
Birds have an internal clock that tells them when to migrate, and they inherit from their parents a general sense of which direction to fly. However, that cannot explain their ability to return to the same spot year after year. Visual cues are, of course, one way the birds can find their way. But cloudy weather can obscure the ground, and strong winds can blow them far off course. Some birds also navigate using the stars, but high clouds can block their view of the heavens, too. Many birds also use an internal magnetic compass that is linked to their eyes that allows the birds to “see” the correct route. A bird’s sense of the earth’s magnetic field is far more sophisticated than the compass we used before the invention of the GPS. A recent article in Scientific American explains the molecular-level changes in birds’ retinas that allow them to “see” the Earth’s magnetic field and use it as a means of navigation.
The evolution of navigation abilities is only one of the astonishing ways birds have evolved to migrate successfully. Some birds shrink and grow their internal organs twice a year to prepare for migration. Bar-tailed Godwits, the champions of nonstop migration, absorb as much as 25 percent of their liver, kidneys, and digestive tract in order to be able to make room for the fat that fuels their 6,800-mile migration from Alaska to New Zealand. Swainson’s Thrushes “sleep” for 9 seconds at a time during their 3,000-mile migration. Ruby-throated Hummingbirds, which weigh less than a nickel, double their weight before flying 500 miles nonstop across the Gulf of Mexico.
A National Geographic article shows that Arctic Terms also are made for migration. They fly 28,000 miles from Greenland to the Weddell Sea in Antarctica and back again each year for their entire lives, equivalent to three round trips to the moon over a 30-year lifespan. That distance doesn’t include the side trips they make along the way following the best weather and food sources. They are very lightweight, so they let ocean breezes carry them great distances without having to use a lot of energy flapping their wings. They also can sleep and eat while gliding.
If you’d like to see the Tundra Swans while they are wintering in Virginia, the best nearby place to visit them is from the Woodmarsh Trail in the Elizabeth Hartwell Mason Neck NWR. Try to go when the tide is high, so the birds are closer to shore. You can check the tide table here. Visibility is best in the morning when the sun is behind you.