- The Washington Times - Thursday, November 14, 2002

A 3-year-old white-throated sparrow, after nesting up north, has come back to the exact same wooded spot in the Patuxent Research Refuge three winters in a row to escape freezing temperatures and munch on the berries of Hercules'-club, arrowwood and bittersweet.
"This white-throated sparrow has been captured in nets about 50 meters (about 50 yards) apart, or less," says Deanna Dawson, research biologist at the Patuxent Wildlife Research Center.
Ms. Dawson captures birds in nets and puts numbered bands around their legs, which enables her to identify them later and track their movements.
The sparrow has band number 1591-88506 and has wintered three consecutive years at Patuxent. It obviously likes its winter quarters, but how does this tiny bird find its way to the wooded spot in Patuxent from its nesting grounds, which might be in New York or as far north as Canada?
"How birds find their way is still one of the most interesting questions in animal behavior, if not all behavior," says Sid Gauthreaux, professor of biological sciences at Clemson University in Clemson, S.C.
Researchers know a lot about the behavior of birds that migrate seasonally, but not everything. The biggest enigma is still how the birds find their way back and forth to the same exact spots, whether their nesting and wintering grounds are a few hundred miles apart or thousands of miles apart.
"We still don't know the exact physiological mechanisms birds use to find their way," Mr. Gauthreaux says.
• • •
Researchers do know, however, that birds somehow are able to use a combination of navigational methods, including using landmarks, magnetic fields, star patterns and even their sense of smell, Mr. Gauthreaux says.
"What's been proposed is that birds use all these cues, but that there is a hierarchy to the cues," he says. "Some of the cues get them in the ballpark of the location, and others get them in the specific location."
Using Earth's magnetic fields probably is one of the "ballpark" cues, Mr. Gauthreaux says. "But it might require something that is much more precise than magnetic fields to get you to that exact piece of real estate."
So, when a bird has found the general area that it is seeking, it may use more specific landmarks, such as trees, rivers or mountain ridges as its "specific location" cues, Mr. Gauthreaux says.
"There is evidence that birds are very keen on using landmarks," he says. "Birds have incredible site fidelity."
Then again, other research suggests that some birds have a great sense of smell, Mr. Gauthreaux says. "There are homing pigeons who find their way back to the same loft even if they are blindfolded," he says.
Stars also provide birds with ways to determine their location and where they want to go. "There is little doubt that star patterns are important in defining the axis of migration," Mr. Gauthreaux says.
This means the stars and their constellations help birds figure out what's north, what's south, what's northeast and so on. How exactly they determine this is still unclear, he says.
Scott Sillett, a biologist at the Smithsonian Migratory Bird Center, agrees.
"There is still no final answer as to how they find their way," Mr. Sillett says, but he adds that there must be some type of genetic component to a bird's ability to find its way.
"They know [from birth] where they are going to go. They don't say, 'I think I am going to California instead of Jamaica,'" Mr. Sillett says. If their forefathers went to Jamaica, that's where they are going, too.
All these cues and genetic components play into birds' ability to migrate, but how exactly birds use these tools is still unclear, Mr. Gauthreaux says.
"You can respond to something that you are sensitive to, but how you use it is a different question," he says.
• • •
Though human knowledge of how birds find their way to wintering and nesting spots is still limited, it is fairly clear why birds migrate.
"Most birds migrate to escape seasonal adversity. When they become hard-pressed to find insects, they leave," Mr. Gauthreaux says. "They migrate because they are looking for more food. That's the biological reason."
A lot of the colorful little migratory birds, such as songbirds, come north from the Caribbean, Mexico or Central and South America only to breed. They need more food when they breed than at other times of the year, and the mid-Atlantic area provides a "superabundance" of food in spring, Mr. Sillett says. The birds' favorite is caterpillars.
It takes these small birds about six to eight weeks to make the trip, but they can travel a hundred miles or more in a day.
Migratory birds have higher productivity than "sedentary" birds, which explains the drive to go north to breed.
Migration comes at a cost, though.
Sedentary birds that know their environment such as where their predators are are less likely to die an early death than migratory birds, which encounter all kinds of dangers along the way, Mr. Sillett says.
The farther the bird has to go, the more likely it is to have a fatal encounter, he says.
Hazards can include bad weather, such as hurricanes, predators, cell-phone and other communication towers and a lack of food.
"A lot of birds migrate at night," Mr. Sillett says. They primarily use the stars to navigate, but other lights attract them, too, in particular the red lights of communication towers.
"You can sometimes find hundreds of dead birds below communication towers," Mr. Sillett says. "They fly into the wires and die."
It is difficult to estimate what the mortality rate is during migration, Mr. Gauthreaux says, but it might be as high as 80 percent.
"That means that only 20 percent of the birds have to generate the next generation," he says. "Migration is a costly behavior, but birds continue to do it. So it must be worth it."
• • •
The climate changes that have occurred in the last few decades can be another challenge for migratory birds, Mr. Sillett says. It's still unclear how the climate changes will affect birds long-term.
One theory is that it might make migratory birds sedentary, Mr. Gauthreaux says.
If the whole globe heats up enough to mimic the temperatures in the tropics, there may be no real appeal for birds to migrate anymore, he says.
In the meantime, millions of birds still migrate every spring and every winter.
In the District and surrounding areas, bird-watchers can see the following migrants (among many others) who winter here: hermit thrush, fox sparrow, white-throated sparrow, dark-eyed junco and golden-crowned kinglet.
Ms. Dawson, who has studied bird migration for about 20 years, is still amazed at these birds' ability to find their way back and forth between nesting and wintering habitats.
The white-throated sparrow that she has seen for three years in a row might have traveled anywhere from 200 miles to 1,500 miles (its nesting grounds are unknown and could range from Pennsylvania to north of the border with Canada) to land in that small wooded spot of Ms. Dawson's Patuxent study area.
"There are so many hazards," she says. "Some of them crash into buildings, some of them experience bad weather. But many make it back year after year. When you see how small some of them are I find it really impressive."

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