Animal migration is one of the greatest mysteries on the planet.
By Brittany Grayson
Each year, countless animals set out from their homes, in some cases the only place they’ve ever lived. Without atlases, compasses, GPS units, or interstates, they fly, swim, or walk hundreds or sometimes thousands of miles to an entirely new location, usually in a prime tropical environment. Later, for reasons still not fully understood, they head back to where they started. Their flapping wings, slapping fins, and plodding feet have echoed up and down the globe for millennia. And for as long as there have been humans, we’ve wondered how they do it.
Migratory black-crowned night herons make their home—and lay eggs—at the Zoo every year. (Jessie Cohen/NZP)
Most scientists define migration as a repeated seasonal movement where an animal leaves one location, moves to another, and then returns to the first location, whether those two locations are separated by several miles, several thousand feet of elevation up and down a mountain, or several thousand miles in journeys that
“There isn’t a really clear definition,” says Pete Marra, a scientist at the Zoo’s Smithsonian Conservation Biology Institute (SCBI). “It depends on the organism. If you’re talking about a small salamander, it may only move 500 yards, but when it goes back, it’s a migration.”
According to Russ Greenberg, migratory bird scientist at SCBI, almost all North American birds migrate, with the exception of chickadees and their relatives, jays, crows, and woodpeckers. Greenberg studies birds called neotropical migrants: small songbirds that summer in North America and fly down to Central and South America for the winter. One of the best known neotropical migrants is the Baltimore oriole, which spends its winters in Central America.
Migratory songbirds follow the most traditional pattern of migration. They are born and spend the first season of their life in the temperate northern summer. Then, as the days get shorter and colder, they start displaying a trait that scientists call Zugunruhe, or “migratory restlessness.” They begin building up their fat stores and eventually take off for the journey south.
The birds fly until they hit warmer climates, where they’ll spend the winter. Some fly directly to the same locations where their forebears spent the winter, hitting the mark like an arrow each year. Others, which Greenberg refers to as “facultative” migrants, are more flexible. They leave their summer grounds only when food becomes scarce, and they lack a specific destination. They’re just looking for somewhere warm and with plenty of food, and they fly only as far south as they need to.
Once spring comes around again, why do animals leave the winter grounds at all? Food is abundant, the weather is warm, and there is no risk of dying on a long journey. The answer seems to be that birds that migrate north face less competition and nest predation than birds that stay in the tropics all summer.
Birds that migrate to temperate zones in the summer are able to take advantage of abundant food and extended daylight hours in which to forage, good things for birds that want to raise a lot of offspring. According to Greenberg, “They could survive as well in the tropics, but they couldn’t reproduce as well.“ He says that, on average, birds that migrate produce twice as many offspring as a species in a similar ecological niche that doesn’t migrate.
Can migratory animals live successfully in a zoo? The answer, it turns out, is yes. Don Moore, head of animal care sciences at the Smithsonian’s National Zoo, explains, “Migration, along with hibernation, is an adaptation to a shortage of food. In zoos, animals are healthy, and they have plenty of food. So they don’t have to migrate.”
Sam, a bald eagle at the Zoo. (Mehgan Murphy/NZP)
That was the case with Sam, a bald eagle at the Zoo. She was from an Alaskan subspecies that migrates in the wild, flying over open water. Yet the eagle exhibit provided everything she needed, including a carefully balanced diet. So she never manifested any migratory restlessness.
Even in zoos, however, some animals do feel the instinctual pull to migrate. Scientists in Germany studied songbirds in cages, and it turns out that, at the time of year when they’d start to migrate, they begin to feel migratory restlessness.
This was demonstrated by the fact that they spend more of their time hopping along the wall that faces the direction they want to go. Greenberg even says that, in species where the birds fly a predetermined distance in one direction and then turn another way, the birds will actually vary the direction of their hops to match the migration route they’d follow in the wild.
Regardless of what causes the urge to migrate, Greenberg says, the animals do not appear very upset about not being able to migrate. It seems they are just responding to their internal rhythms. Greenberg explains, “They don’t bash themselves on the side of the cage if they can’t migrate. They just hop around a little more.”
Some of the migratory animals at the Zoo aren’t in enclosures; they are just passing through. Tamie DeWitt studies perhaps one of the most charismatic and phenomenal migrants: the monarch butterfly. The Zoo has butterflies in the Pollinarium attached to the Invertebrate Exhibit, but there are no monarch butterflies in there. Monarchs are high-fliers, not suited for living in an enclosure.
Monarch butterfly. (Jessie Cohen/NZP)
You can still find them at the Zoo, though. All summer, they flit around the grounds looking for mates, flowers from which to sip nectar, and milkweed on which to lay their eggs. DeWitt explains that every butterfly species has one species of plant it has to eat when it first hatches out of its egg. For monarchs, that’s milkweed. DeWitt says sympathetically, “It’s unfortunate that it’s got that word weed in its name, but there are some really pretty milkweeds.”
All throughout the summer, monarchs hatch out of eggs on milkweed plants, pretty or not, as caterpillars. The newly hatched insect eats until it grows large enough to build a chrysalis, where it transforms into a butterfly. After it emerges from the chrysalis, its wings dry and harden in the sun. Then, for most of the summer, the butterfly flits off in search of two things: mates and nectar.
Something changes in September. DeWitt explains, “The monarchs that hatch out in June, July, and August just want to mate. The generation that hatches out in mid-September doesn’t want to mate. They look to migrate.” This migrating generation is different from all the generations that preceded it that summer. These butterflies are sturdier in build and structure, and their first priority seems to be to eat as much food as possible and then to fly south.
DeWitt studies butterflies by tagging them. When she catches an ordinary monarch in the summer and releases it, it wanders off in any random direction. But when she tags and releases a migratory monarch, it always flies the same direction—south. This marvelous insect manages to fly almost 2,000 miles, roosting on trees at night, to a mountainous area southwest of Mexico City. Sometimes monarchs fly individually; other times they form enormous flocks called “clouds.” Yet they all make it to the same area: a mountainous region where they roost in the Mexican oyamel pines, insulated by the trees, for the rest of the winter.
Come spring, when the days lengthen again, these same butterflies fly back north. They make it to the Gulf states, where they immediately begin to reproduce and die. Subsequent generations finish the return journey to the northern United States and Canada. The whole cycle is an amazing feat for creatures weighing about half a gram.
Perhaps the biggest question scientists have about migration is how animals know where to go. Some steer by the stars. Greenberg says you can put a bird in a planetarium, and the bird will fly the correct direction as pointed by the stars. They learn the star patterns as youngsters. Many birds seem to also have a built-in compass that guides them, unerringly telling them which way north is.
Migration is difficult to study, because it takes place over such great distances, and because it involves so many apparently independent decisions by so many individual animals. SCBI scientist Peter Leimgruber studies migration and movement patterns in endangered species, including gazelles and elephants, all over Asia. He and SCBI statistician Justin Calabrese are working together, collating data from tracked migratory animals that range from ungulates (gazelles and deer) to cranes. Using these data, they’re hoping to unlock the secrets of how animals decide to migrate, and how they navigate once they’re on the move.
Leimgruber says that one of the most interesting things about animals that move long distances is puzzling out which animals truly migrate, in a repeated pattern, and which are merely nomadic, engaged in a random search for seasonally shifting food resources. “The fact is we really don’t know much about migration,” he says. “The difficulty is to track individuals on their paths.”
So far, most of the migratory species studied have been the most “predictable” migrants, Greenberg explains. “We study birds that are easy to study; birds that are territorial in the winter, and that we can find again.”
New technology is changing that. Scientists like Leimgruber are now able to monitor large, unpredictable mammals by tagging them with satellite collars. In this way, they can track animals over long distances, with unpredictable movement patterns, from the comfort of a computer.
Boosting our understanding of migration is crucial for wildlife conservation. Most conservation efforts focus around protecting selected areas of land. With migratory animals, this isn’t enough. The challenge is getting the governments of many different countries to work together. And not only that, but the changing climate is shifting migration patterns and timing. The more we know about migration, the better a chance we have of protecting animals. Leimgruber sums it up: “We’re trying to protect moving targets.”
—BRITTANY GRAYSON is a web editor and science writer for Friends of the National Zoo.
If you have a comment about Smithsonian Zoogoer magazine, please email it to us.Smithsonian Zoogoer 40(4) 2011. Copyright 2011 Friends of the National Zoo. All rights reserved.