The Challenges of Maintaining 2 Homes

You may struggle with a long commute, but it can't be as bad as the blackpoll warbler's.

ALSO SEE: Life on the Wing

The 5.5-inch-long bird, which weighs no more than three nickels, flies from its breeding grounds in Alaska and northern Canada to the western Amazon in Brazil and Bolivia, where it spends the winter. It takes the scenic route, visiting New England, the western Atlantic, and Venezuela or Guyana along the way.

Other birds make similarly long treks twice a year, but the distance award goes to the arctic tern. The robin-size seabird breeds in the Canadian Arctic and then spends several months flying down the west coast of Europe and Africa to Antarctica -- a total of about 10,000 miles each way.

The adaptations that allow birds to undertake such lengthy journeys and to know when and where to go have long fascinated ornithologists. Before leaving, birds in the wild eat like gluttons and layer on fat to provide sustenance during their migration. They depart only when weather conditions are favorable. They navigate using landmarks, the sun and stars, and perhaps even the earth's magnetic field.

Scientists have only recently looked beyond that incredible journey and recognized another important aspect of migration: that birds must adapt to a commuter lifestyle. They must adjust to new climates, diets, and predators every time they move from their winter to their summer homes.

"You've got a bird, like an ovenbird, that breeds in Maryland for three months of the year. But for most of the year, that bird can be in Mexico or Belize, in a tropical environment, eating very different insects," says Peter P. Marra, a terrestrial ecologist at the Smithsonian Environmental Research Center.

At a symposium on migratory birds organized by the Smithsonian Institution and held here last month, researchers described early work on the adaptations that birds may have evolved to deal with their dual citizenship. Birds, scientists are learning, must alter their digestive systems to deal with new foods, must balance when to molt with the weather and food patterns of their habitats, and even may have evolved novel behaviors and cognitive abilities that allow them to explore unfamiliar areas every year.

William H. Karasov, a professor of wildlife ecology at the University of Wisconsin at Madison, was curious about how switching from one specialized diet to an entirely different one affects birds. "American robins, if you watch them during the summertime, they eat worms and arthropods. You'd never think they were fruit-eating," he says. But during the winter, "80 or 90 percent of the material in their guts is fruit."

Nouvelle Cuisine

He guessed that the birds' digestive systems might be able to handle any food. But that's not what he found. "If a bird is eating one type of food, like an insect diet, which is protein and fat, and you suddenly switch them to a fruit diet, they are inefficient. They cannot extract all of the simple sugars," he says.

But given a little time, they can adjust. The five species that Mr. Karasov studied alter the type of digestive enzymes they produce and how long food remains in the gut. In robins, the switch takes about two to three days. That's quick enough for birds to get sustenance during their migratory journeys; many stop for a week or so in different environments to feed and rest along the way.

Mr. Karasov's discovery is important, says Russell Greenberg, director of the Smithsonian Migratory Bird Center and an organizer of the symposium along with Mr. Marra. "Migration oftentimes has to do with dietary change, but ecologists have not appreciated that animals can't just be hopping around eating fruit one minute and insects the next."

What's unclear, however, is whether birds' digestive changes are an adaptation to migration. Many birds that don't migrate switch diets during the year as food supplies vary. "Whether it occurs in proportionally more migrants, boy, I'm not sure," says Mr. Karasov.

The Incredible Shrinking Gut

Some birds also undergo a far more spectacular change having to do with food. Because of the long fasts they endure during migration, their guts shrink by half or more to reduce energy consumption. Their guts then grow after they begin eating again.

Not all birds who do that are migrants. Penguins, for instance, shrink their guts during a prolonged fast in the breeding season. But migratory birds often shrink their livers, stomachs, or intestines even while feasting in anticipation of their fast, not just during it.

Theunis Piersma was the first to discover that change. "The paradigm was that birds are like planes. They refuel, they just store fat, and then they burn the fat," says Mr. Piersma, an ecologist at the University of Groningen and the Netherlands Institute for Sea Research.

But he discovered in the mid-'90s that, before migrating, shorebirds called bar-tailed godwits eat enough to make fat account for more than half of their body weight while also shrinking their viscera by half. In experiments, Mr. Karasov later found that caged yellow-rumped warblers feeding at a regular rate but subjected to long days and short nights -- which made them restless, as though ready to migrate north in spring -- also had guts 40 percent smaller than birds kept on normal, winter daylight hours.

A Question of Timing

Scott R. McWilliams, an assistant professor of wildlife ecology at the University of Rhode Island, and Mr. Karasov also reported at the symposium that birds take a couple of days to readjust to eating when their bellies have shrunk during an experimental fast. Mr. Piersma has found through ultrasound studies of shorebirds called red knots that it takes their stomachs four to five days to double or halve in size.

Timing is key to migratory birds' survival and success on the breeding grounds. "Predation risk is probably directly related to how long you're at [stopover] areas," says Mr. McWilliams. "And if a bird misses out on leaving with favorable winds, it may have to spend many more days in migration." As a result, it may arrive at the breeding area too late to stake out the best territory for producing offspring.

No one knows whether gorging on food while shrinking the gut is something only migrants can do. Few scientists have looked at the gut-shrinking abilities of birds that don't migrate because their annual cycles are not so sensitively timed.

Another event in a migratory bird's year with critical timing is molting, says Sievert A. Rohwer, a professor of zoology and curator of birds at the University of Washington's Burke Museum. Most birds in North America molt in the late summer, once they're finished breeding but before they migrate.

Molting takes a lot of energy, as birds grow new feathers to replace worn-out ones. Still, "it's the great neglected area of avian life histories," Mr. Rohwer says. Because birds are replacing feathers, they're not as good at flying. And the partially grown feathers are more susceptible to damage. During molting, he says, birds "are very secretive and don't move much." That makes molting difficult to study. (It doesn't help that molting coincides with many scientists' vacations. That conflict between the annual cycles of birds and academics may be as great as the practical difficulties, Mr. Rohwer observes.)

But some studies have been done, and Mr. Rohwer has detected trends from published work. At the symposium, he presented a comparison of eastern and western North American birds. Of 55 species of birds in the East, 50 molted on the breeding grounds before migrating. But in the West, fully half of 26 species molted after they had left the breeding grounds.

He thinks that the western birds have adapted to the harsh conditions of hot, dry summers by escaping to areas with more plentiful food and less dangerous environs before they molt. Many of the birds stop to molt during their migration in the southwestern United States, where monsoon-like rains provide a flush of vegetation and, presumably, food in the late summer. Others delay molting until they have arrived at their wintering grounds.

Bigger Bird Brains?

Among the most novel ideas presented at the symposium was that migratory birds have evolved new cognitive abilities. Claudia Mettke-Hofmann, a postdoctoral researcher at the Max Planck Research Center for Ornithology, in Andechs, Germany, speculated that migratory birds have developed better memories and more efficient exploration skills than birds that stay in one place year-round.

She described the different challenges that young birds face during their first year of life. All birds must become familiar with the area where they were hatched to search out future breeding sites. But only migrants need to find the breeding grounds again upon leaving. Experiments have shown that fledgling migrants explore a larger area than residents, perhaps in order to find landmarks for their return trip.

When autumn arrives, migrants encounter many unfamiliar habitats during their long trips. No one has tested whether migrants are less afraid to enter novel areas, according to Ms. Mettke-Hofmann. But she has done experiments to see how quickly birds explore new territory. She hid food in one of two rooms and then released either a garden warbler or a Sardinian warbler into the setup. The two species are close relatives, but only garden warblers migrate. She found that garden warblers took half as much time as their Sardinian cousins to discover the cache, and did so by visiting more areas of the rooms in a shorter time.

In a migratory bird's second year, remembering the locations of stopover sites would be a "huge advantage" in speeding the trip, Ms. Mettke-Hofmann says. A long-term memory should be less important for resident birds because they don't stray long from their homes.

To test the idea, she used the same two-room setup, always putting the food in the same room, and tested birds at intervals some time after they first had explored the rooms. The garden warblers typically explored the room with the food first, demonstrating that they remembered where the food was as much as 12 months after they last had seen the experimental setup. But after any interval longer than just two weeks, the Sardinian warblers seemed to have forgotten where the food was, exploring the two rooms equally.

Ms. Mettke-Hofmann has not published her work yet and is wary of providing more detail about her experiments. She acknowledges that generalizing her findings to all migrants and residents based on only one species of each would be premature. "This is a first approach, and with these results, we can continue with other species," she says.

Mr. Greenberg, of the Smithsonian, calls Ms. Mettke-Hofmann's research tantalizing. "It's amazing how little we look at cognitive mechanisms in what is an incredibly big problem: birds moving around and facing different resources," he says. "I really think that's going to catch on and be an important field in the future. ... She took a really bold first step with that experiment, and one can only hope that there'll be a lot more work."

Indeed, Mr. Marra hopes the symposium will spawn new research on the challenges birds face in maintaining two homes. "A lot of people still study their systems on the breeding grounds without really considering what's going on on the wintering grounds," he says. "I would say that they're missing a big part of the picture."

LIFE ON THE WING The blackpoll warbler and the arctic tern migrate enormous distances from their breeding grounds to their winter homes.

Arctic terns » see more daylight hours in a year than any other creature on earth. » spend most of their lives flying. » feed and even sleep on the wing. Blackpoll warblers » cannot land on water. » fly nonstop for 80 hours from New England to South America. » fly at an altitude of 16,000 feet during migration.