Wherever there are birds, there is migration. That birds of all sizes – from giant cranes to tiny hummingbirds – journey thousands of miles twice a year has always fascinated us. We have wondered why birds migrate, how they store enough energy for the trip, and how they find their way.
Why birds migrate seems easy to answer. Who wouldn’t want to avoid a Canadian winter by spending some time in the tropics? But that doesn’t explain why they leave the tropics in the first place. Why does a bird fly a thousand miles north just to build a nest when it could build the nest where it spent the winter? The complete answer to that question is complex but mostly boils down to overcrowding and food supply. By flying north, birds can exploit the wide open, food-rich, bug-infested wilds of North America.
North America provides excellent breeding grounds, which produce millions of baby birds. And it is these baby birds that make migration really amazing. Baby birds, not much more than a month old, somehow find their way south to their species’ wintering grounds. And they do it without the help of their parents or a map.
The young birds spend the time immediately after fledging playing with their fellow nestlings, honing their flying skills, and learning how to find food. Then suddenly they are struck with a case of Zugunruhe – that’s German for “migratory restlessness.”
Instantly, the fun is over and they start getting antsy. They eat constantly and nearly double their body weight. Then late one night, when the weather is just right, they fly into the black night sky and away from the only world they know. Using the stars, the earth’s magnetic field, and instinct inherited from their parents, they stream south. Some of them make short flights, stopping along the way to eat and rest; others fly nonstop. Blackpoll Warblers, for example, take off from Nova Scotia, fly hundreds of miles out over the Atlantic Ocean, turn right just east of Bermuda, and fly 86 hours nonstop to South America.
Bird migration is as diverse as it is amazing. The cardinal pretty much stays in the same place all year, and the Snowy Owl only migrates when there is a food shortage. The Bobolink can’t get far enough south; it flies all the way to Argentina. The 4-ounce, 15-inch long Arctic Tern nests in the high Arctic but likes to spend its winter way down in Antarctica. It leaves North America in late summer and flies east across the Atlantic to Europe. Then it works its way south along the coast of Africa and finally on to Antarctica. By the time it completes the annual roundtrip it has flown nearly 22,000 miles.
Equally diverse is the time of day that different birds make their flights. There are birds that only travel at night, others that fly only during the day, and some that don’t seem to care when they fly. Most songbirds fly at night, which is unusual because the rest of the year they roost at sunset. Flying at night protects them from hawks and falcons that migrate during the day. Ducks and geese migrate either day or night.
Nighttime migration fooled many early naturalists who thought birds hibernated in a secret underground hideout. Other observers believed that birds would fly off to the moon for the winter. And once upon a time people thought hummingbirds were not strong enough to fly across the Gulf of Mexico – and so rode the backs of larger birds across the Gulf.
A combination of length of day, temperature, and weather triggers the urge to migrate. When all conditions are just right, the birds make their move. Huge flocks of them go for it at the same time. Radar has detected as many as 15 million birds flying south over Cape Cod on a single night. Can you imagine 15 million birds overhead? That would be the wrong night to forget to take your laundry off the clothesline.
For those of you who don’t have radar, here is something fun to try. Around 11 p.m. on a clear, moonlit night in September or early October, focus your spotting scope or binoculars on the moon. You will be surprised how many birds you may see passing the moon on their way south. I tried it and saw hundreds of silhouettes thousands of feet up in the night sky. Give it a try. Migration is happening now.
In the meantime practice saying, “Zugunruhe.” You know you want to.
Tate is a Carrollton resident and bird enthusiast.