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The audience of Maryland birders was quietly impressed with the author’s slide show, murmuring appreciatively at his close-ups of elusive warblers and fiery-eyed hawks. Then, one image brought an astounded gasp.

Eye in the Sky

It was not of a bird, but of a radar image showing southern Texas and the Gulf of Mexico largely covered by a red-and-yellow blob oozing south. The blob, he explained, represented a sky full of birds migrating south toward Mexico one fall night.

“People are always impressed with this slide,” said Scott Weidensaul, author of Living on the Wind: Across the Hemisphere with Migratory Birds. “It really makes the point.”

Indeed. Migration is much bigger than a flock of geese against the sunset or a group of birds showing up where they weren’t the day before. It’s thousands—even millions—of birds in the air at once. And this year they are being tracked as never before.

A pilot radar ornithology program called Birdcast is providing online forecasts of bird migrations through the mid-Atlantic area. Each day during the spring and fall migration seasons Birdcast posts a prediction for mass bird movement between dusk and midnight, the period when bird migration peaks for the day.

The Birdcast Web site also provides radar images of birds moving through the area during the previous evening. These images, which are downloaded from radar stations operated by the National Weather Service, show the accuracy of the previous day’s forecast.

“We are predicting amazingly well,” says Sidney A. Gauthreaux, who heads the project, based at the Clemson University Radar Ornithology Laboratory in South Carolina. “Of course, our forecasts are dependent on the accuracy of weather forecasts.”


Season, of course, is the biggest factor in bird migration. But second is weather, and particularly wind direction. Optimum conditions for northbound migration occur when there are helpful tailwinds from the south. For southbound migration, when winds are from the north.

“When there are really good migration conditions, everything migrates,” says Gauthreaux. “Even if a bird is low on energy it can migrate a couple hundred miles on a strong tailwind.”

Temperature, moisture and barometric pressure also play a role. Birds are urged northward in the spring by rising temperatures, increasing moisture, and decreasing pressure. Opposite conditions—falling temperatures, decreasing moisture, and increasing barometric pressure—urge them south in the fall. Migrating birds tend to avoid storms and flying against the wind.

Migration models have been created using information about how birds react to various weather conditions. By plugging current weather conditions into those models, Birdcast can make its predictions for the next day’s movement. So far, the radar images are confirming that the models are generally valid, says Geauthreaux. “Theory is one thing, but it’s satisfying to see empirical data.”

Birdcast began migration forecasts earlier this year during the two-month spring migration season, covering the area between Washington, D.C. and Philadelphia. Coverage was expanded north during the fall season to include upstate New York.

“There is lots of interest in taking the program national,” says Gauthreaux. “But that would be very labor-intensive.”


For now, the Birdcast program is in a demonstration phase as scientists work to develop better models. However, Gauthreaux sees practical uses for migration forecasting in the future. Pesticide applications could be avoided in areas where migrating birds stop to rest, and aircraft could be warned when to steer clear of flyways, saving the aviation industry millions of dollars each year in repairs.

But radar images have their shortcomings. One type of radar image can detect whether substances in the air are biological. Another type determines how fast and in what direction they are moving. But radar cannot tell the species of the bird, information that would be especially helpful to pilots. Birds the size of geese can do far more damage than tiny songbirds.

This issue is being addressed with another technology, called bioacoustics. Cornell University’s Lab of Ornithology, another Birdcast partner, has placed ultra-sensitive microphones in the forecast area that can detect the individual calls of migrating birds as high as 11,000 feet (3,353 meters). The sounds are fed into a computer that runs automatic acoustical transient detection software and analyzed to determine which species made them.

The ability to predict migration by species may be a while coming, but that is the goal. The time may come when birders will be able to check a migration forecast to find out what kind of field trip they’ll have the next day.

“It takes a bit of the serendipity out of it,” says Weidensaul. “But I’m willing to give that up for the conservation implications.”

Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at

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•  Migrating birds are guided by a variety of beacons: the moon, sun, Earth’s magnetic fields, topography, and their own sense of smell.
•  Birds make use of tailwinds to save energy on long migrations.
•  Arctic terns are the long-distance migration champions, making two 10,000-mile (16,093 km) trips a year between the extremes of the northern and southern hemispheres.
•  Ruby-throated hummingbirds, weighing only 4 grams, migrate 500 miles (805 km) nonstop across the Gulf of Mexico.

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A long-distance migration is every bit the challenge for birds that it seems. Winds blow them off course, storms dash them into the sea, predators target exhausted birds as they land. Natural forces have always taken a toll.

But today, humans are the biggest threat to migrating birds. Forest fragmentation, suburban sprawl, and wetland drainage destroys critical stopover places where birds feed and rest. Pesticides wipe out insect prey. Millions of outdoor cats stalk exhausted migrants as they land. And increasingly, migrants are killed in collisions with communications towers and tall buildings. By some estimates, migration numbers are down to half what they were 50 years ago.

The first step to preserving habitat for migrating birds is finding out where they land to rest and feed, says Scott Weidensaul, author of Living on the Wind: Across the Hemisphere with Migratory Birds.

“Radar ornithology,” he says, “is helping us find out where the birds want to be.”