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Migratory Marvels

New lightweight devices are revolutionizing the tracking of migratory birds—and documenting surprising new records for distance and physical endurance

  • Jessica Snyder Sachs
  • Mar 15, 2011

ON A CRISP MORNING last April, John Tautin focused his spotting scope on one of 74 apartment-style nesting boxes that stud the Purple Martin Conservation Association’s breeding colony near Pennsylvania’s Edinboro Lake. “That female at compartment 52 has a geo,” he called to his wife, wildlife biologist Joan Galli.

Galli and Tautin, the association’s director, had been scouting for spring arrivals all week. Looking from a different angle, Galli confirmed her husband’s sighting and added her own. “That male has a geo, too!” she said of a second martin landing on the same perch.

On each martin’s back, peaking just above its feathers, was the short, slender stalk of its “geo”—or geolocator, an animal-tracking device recently scaled down to songbird size. Weighing just 1.1 grams, or less than two M&Ms, the device does one thing: record maximum light intensity at 10-minute intervals. When retrieved, a year’s worth of such light data can be computer analyzed to trace a bird’s day-by-day location—using the time of sunrise and sunset to determine latitude and longitude.

In 2007, Tautin’s group helped biologist Bridget Stutchbury of Ontario’s York University attach geolocators to 20 purple martins in northern Pennsylvania. The following year they retrieved the devices from two returning birds. Along with five geo-tagged wood thrushes recaptured by Stutchbury’s students, the Pennsylvania martins became the first songbirds ever tracked through their entire migration.

Data from the geos pinpointed the birds’ Amazonian wintering grounds, crucial information for guiding conservation efforts. The bigger news was how fast they traveled. One female martin averaged 360 miles a day on her 4,600-mile return to Pennsylvania—four times faster than the 90 to 95 miles a day long believed standard for songbirds. And that was with a relatively clunky geolocator on her back. The first year’s poor return rate (just two martins) prompted a redesign to streamline the devices further.

Last April, returns were on track to a more typical 50 percent, when Tautin and Galli spotted the two geo-tagged martins at a single nest box. After the birds flew away from their nesting cavity, the human couple slid the entire box down its pole, attached a shutter above door 52, and propped the shutter open with a pin tied to a fishing line. With the house back in position, the martins returned within minutes and hopped inside the nest. Tautin tugged the fishing line. The shutter dropped. Two geolocators were ready for retrieval.

Today lightweight devices such as geolocators are revolutionizing the tracking of migratory birds and, in the process, documenting astonishing new records for distance and endurance. The first surprising discovery came in 2007, when wildlife biologists used surgically implanted satellite transmitters to show that migrating bar-tailed godwits fly from Alaska to New Zealand without once stopping to refuel. At 7,100 miles in just over 8 days, the migration was, and remains, the longest nonstop flight ever recorded.

Last year, an international team used geolocators to trace the impressive migratory journey of the Arctic tern, confirming that these birds migrate the longest distance of any animal—close to 50,000 miles a year (with stops), or the equivalent of 3 journeys to the moon and back over a tern’s roughly 30-year lifetime.

Now biologists are scrambling to understand the body-transforming physiology behind these ultra-marathon flights. Their findings go beyond “gee whiz” science by helping researchers understand the nutritional needs of long-distance migrants. At the same time, the new ability to track migrants with precision has pinpointed crucial refueling and wintering habitats—many rapidly disappearing beneath large-scale development.

Migratory birds have a long history of defying scientific ideas about their capabilities, says biologist Douglas Altshuler of the University of California–Riverside. Until the 1950s, many experts scoffed at reports that ruby-throated hummingbirds were traveling directly across the Gulf of Mexico on their way from the Yucatán Peninsula to the U.S. Gulf Coast—a distance of 600 miles. Weighing in at a fraction of an ounce, hummingbirds are the world’s smallest birds and also have the highest oxygen consumption relative to their size. “To be burning fuel that quickly makes a 600-mile journey all the more challenging,” Altshuler marvels.

Physiologists had tried to calculate the hummingbird’s limits of endurance by comparing energy intake to output. Outside of migration, the birds operate almost entirely on blood sugar and stored carbohydrates, or glycogen. “For a hummingbird to make it across the Gulf of Mexico on carbohydrates, it would have to beef up to about 12 grams,” says nutrition physiologist James Hargrove of the University of Georgia in Athens. “It wouldn’t be able to lift off its branch!”

In fact, it turns out that hummingbirds have an unusual ability to abruptly flip from metabolizing carbohydrates to fat immediately prior to migration. “They can get across the gulf on roughly one gram of fat,” Hargrove says of “the best use of a gram of fat anywhere in the universe.”

In addition, hummingbird flight muscles are highly unusual. They’re made entirely of just one type of muscle fiber—dubbed FOG for fast oxidative glycolytic. “It allows hummingbirds to move quickly, but fatigue slowly,” Altshuler explains.

Yet even with their remarkable metabolism and high-efficiency muscles, hummingbirds crossing the Gulf of Mexico don’t have much oomph left once they make landfall—particularly if they encounter rough weather along the way. “That’s why patches of forest near the gulf coast are so important to their success,” says Altshuler.

Fuel burning is not the only physiological barrier to the bird world’s ultra-marathoners. “Shipwrecked sailors can survive over a month without food, but will die of thirst in a few days,” says physiologist Christopher Guglielmo of the Advanced Facility for Avian Research at the University of Western Ontario. “Now think about a bar-tailed godwit flying nonstop for almost nine days. How do such long-distance migrants retain enough water to sustain flight? And why don’t we find dehydrated birds at the end of the journey?”

It’s a given that migrating birds virtually shut down their kidneys to avoid losing water. “But they also lose major amounts of water by simply breathing,” says Guglielmo. The drier air encountered at high elevations may exacerbate water loss. This may explain why trans-gulf hummingbirds are often seen skimming just above the waves. The extra humidity right above the water may reduce the rate of water loss, and flying just above the surface can be more efficient aerodynamically, he says.

In addition, Guglielmo and others have found that migrating birds can produce their own metabolic, or internal, water—though sometimes at the cost of consuming their own muscles and even organs. “Burning lean tissue produces a lot more metabolic water than burning fat alone,” he explains.

Naturally, the bird that breaks down too much muscle won’t make it to journey’s end. But breaking down digestive organs may be one way for a long-distance, nonstop migrant to scuttle unnecessary baggage, says biologist Robert Gill of the U.S. Geological Survey’s Arctic Shorebird Program. In the late 1990s, Gill collected nine bar-tailed godwits that had collided with a radar dome off the Alaskan coast just a few miles after starting their southward migration. Autopsies showed that the birds had miniscule gizzards, livers, kidneys and guts compared to a set of godwits that had been illegally poached in New Zealand during the winter.

Without their digestive organs, the departing godwits weighed significantly less than the wintering birds—despite being so fat that Gill likened them to “flying softballs.” Indeed, fat accounted for more than half of the just-launched godwits’ body weight.

This pre-migration breakdown of digestive organs may be even more dramatic in the red knot, a crimson-fronted sandpiper that migrates from the Arctic to New Zealand or South America with stopovers as many as 5,000 miles apart. During their southern winters and northern summers, red knots gobble up hard-shelled prey whole, digesting it with a heavy-duty gizzard that, relative to body size, is the largest of any shorebird. But over the course of one week, a red knot can halve the size and weight of its digestive organs, says Phil Battley, a biologist at New Zealand’s Massey University who studies migrating shorebirds.

In other words, when ocean-crossing migrants finally make landfall, they’re not just weak and hungry, they need to literally rebuild their organs before they can digest normally. “And that takes protein,” says Guglielmo. A hummingbird that has just crossed the Gulf of Mexico, for instance, cannot simply tank up on nectar. “It needs insects and lots of them,” he says. So do millions of other gulf-migrating songbirds, including vireos, thrushes, warblers, tanagers and orioles.

“When these birds first make landfall over Texas, they’re looking down on just a few patches of forest already crowded with other birds,” says Altshuler. “If a bird has enough energy left, it’s going to do better if it can keep heading north. But it may be getting harder for them to find the quality habitat they need before running out of fuel.”

For globe-spanning shorebirds such as godwits and knots, refueling depends on mudflats and estuaries rich with shellfish and other invertebrates. Red knots, for example, normally gorge on the eggs of horseshoe crabs when they stop over in Delaware Bay each spring. Unfortunately, the bay’s historic horseshoe crab population is being overharvested as bait for commercial eel and conch traps.

A far greater threat is the large-scale development of fish and shrimp farms that is consuming tens of thousands of acres of estuaries in the Yellow Sea region of China and the Koreas. In recent years, Gill, Battley and others have used tracking technology to confirm that this area is one of the most important refueling stations for the world’s long-distance migratory shorebirds—a vital stopover for hundreds of thousands of sandpipers, plovers and turnstones, including many species seen along the western and eastern coasts of North America. Already, their colleagues in China and South Korea are reporting steep declines in the number of birds passing through the region.

Of particular concern has been the disappearance of vast numbers of great knots, a speckled wader that migrates nearly as far as the bar-tailed godwit. “The whole global population passes through eastern Asia on their migration, and up to a third use this one site,” Battley says of Saemangeum, an estuary-rich region along the Yellow Sea that the South Korean government dammed and drained for fish and shrimp farming, agriculture and other development projects in 2006. “Three years later, the great knots at Saemangeum and neighboring estuaries went from 116,000 to 26,000,” Battley reports. “The hope was that they could relocate to neighboring sites. But most appear to have simply disappeared.”

Meanwhile, satellite images of the Yellow Sea’s Chinese and North Korean shores reveal a patchwork of drainage and development activities that may add up to even greater habitat destruction. “We’re looking down on a latticework of fish farms where there had been hundreds of thousands of hectares of mudflats,” Battley says.

Battley, Gill and their colleagues have mounted an international campaign to raise awareness of the Yellow Sea’s unequaled value to the world’s migratory birds. “We realize that given the state of the global economy, migratory birds are not even on the radar of many government policy makers,” Gill admits. “But they should be, because if we wake up too late there may be nothing left to save.”

Jessica Snyder Sachs is a New Jersey-based writer and frequent contributor.

Migrants Threatened By Global Warming

Being out of sync with climate change may put long-distance migratory birds at a distinct disadvantage. The reason? While short-distance migrants appear to adjust their schedules in response to temperature shifts, birds traveling between continents or even hemispheres continue to time their migrations by seasonal changes in day length. The result is a “growing mismatch in the arrival of short-distance and long-distance migrants on breeding grounds, where competition for scarce resources such as nesting sites can be fierce,” says Stanley Temple, professor emeritus of wildlife ecology at the University of Wisconsin–Madison.

Some of the clearest evidence comes from Temple’s comparison of springtime events recorded by the famed conservationist Aldo Leopold (Temple’s predecessor at the university) and Leopold’s daughter Nina. Both kept meticulous records at Aldo Leopold’s famous “shack” in central Wisconsin. But they did so decades apart—Aldo from 1935 to 1948 and Nina from 1976 to the present.

The Leopold diaries reveal that as average seasonal temperatures have risen steadily since the 1930s, short-distance migrants such as robins, sparrows and blackbirds have advanced their arrival times a week or more. But long-distance migrants such as flycatchers, thrushes and warblers have remained on their traditional schedules.

This puts the great crested flycatcher at a particular disadvantage, Temple says. “It’s one of several species that nests in pre-existing cavities but can’t make its own, and some of its most serious nest competitors, such as starlings, are now arriving as much as two to three weeks earlier than it is.” Wisconsin’s great crested flycatchers have declined by nearly a third since the 1960s, Temple notes. “I wouldn’t leap to blame a climate change tilt in nest competition entirely,” he says, “but it certainly isn’t helping.”


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