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Whooping cranes will be tracked with GPS units to study their habits and hazards

By David Brown
Washington Post Staff Writer
Tuesday, March 30, 2010; HE01

WOOD RIVER, NEB. -- Each dawn and dusk, numberless birds stopping here to feed on their migration north take to the air. Against the steel-colored sky they look like iron filings wheeling and milling to an invisible magnet.

Most are sandhill cranes, whose beauty and marionette-like dance draw bird-watchers from across the country each spring. Some are snow geese, dabbing the dun fields with their white bodies. Teal, pintail, mallard -- and dozens of other species of waterfowl -- pass through in thousands. On the avian interstate known as the Central Flyway, Nebraska straddles the middle lane.

Somewhere among the flocks over the next few weeks will be a small number of whooping cranes. Huge white birds with red crowns and black legs, they will be flying in twos and threes, with rarely more than a dozen congregating on the brief sojourn here en route from Texas to Alberta.

At five feet, they are the tallest birds in North America and also among the rarest. As of last month, only 256 survived in the wild. They've always been a bit mysterious, too -- which is saying something for a bird that weighs 14 pounds, has a wingspan of 7 1/2 feet and lives 30 years. Flying 500 miles a day at 40 mph 3,000 feet above the ground, they're nature's version of a spy plane.

Although they are ancient inhabitants of the continent -- whooping crane bones have been found in the 38,000-year-old La Brea tar pits of California -- the birds apparently never remotely approached the numbers of their smaller cousins. By one estimate there were fewer than a thousand in the world in the early 1800s. (About 650,000 sandhill cranes come up the Central Flyway today.) Their nesting ground, in a Canadian national park straddling the border of Alberta and the Northwest Territories, wasn't discovered until 1957.

What is certain is that the whooping crane nearly went extinct in the first half of the 20th century, its population bottoming out at 14 or 15 individuals in the spring of 1941. Since then, like an individual whooper taking flight, it's been a long, slow upward climb.

As a consequence, the deaths of even a few whooping cranes get the attention of researchers such as the ones here at the 30-year-old Platte River Whooping Crane Maintenance Trust. This year for the first time, the movements of a few birds are being monitored in real time through miniaturized leg-band Global Positioning System devices.

This research project took six years of meeting, discussing and permit-getting in two countries. It is finally underway, at a propitious time. A census of whooping cranes conducted last spring as the birds were preparing to head north revealed a startling spike in mortality. Fifty-seven had disappeared in the preceding 12 months. Twenty-three of them died on the wintering grounds at the Aransas National Wildlife Refuge, on the Gulf of Mexico about 50 miles northeast of Corpus Christi, Tex. The rest apparently died during migration.

(A smaller population of captive-bred and released whooping cranes in Florida has had different problems. Although some of the birds have been successfully taught to migrate to a breeding site in Wisconsin, almost none produced offspring.)

Every six hours

Wintertime mortality is usually very low in the Texas-to-Alberta wild flock, often only one or two birds. While the reason for the large number of deaths two winters ago isn't known, some may have succumbed from a shortage of food. The Aransas wetlands were notably dry that year, reducing the number of blue crabs, a major constituent of the whooping crane diet.

While sandhill cranes happily feast on waste grain in the muddy crop fields of the Platte River basin, whooping cranes greatly prefer animal to plant material. They eat snakes, lizards, clams and dragonflies (but rarely fish, which they seem to have a hard time catching). Although great bird artist John James Audubon believed incorrectly that whooping cranes and sandhill cranes were the same species, with the sandhill's gray-brown a juvenile coloring, his portrait of the whooper was dead-on. He depicted the huge bird preparing to devour two just-hatched alligators.

"Species that are very specific in their diet are much more vulnerable. That seems to be the case with whooping cranes," said Karine Gil, a population ecologist at the Crane Trust.

What caused the deaths of the migrating birds isn't known, either, although the conventional wisdom attributes most fatalities to collisions with power lines. Nine whooping crane carcasses recovered in recent decades suggested such a death.

But Felipe Chavez-Ramirez, the biologist who heads the Crane Trust's research station, doubts that's the whole answer. "We think that under normal flying conditions the cranes should be able to get around them," he said this month.

He, Gil and their collaborators, however, may be about to get a more complete picture of the hazards of migration, and of the successful features of it as well.

They have permission to trap and band 10 whooping cranes in the winter and 10 in the summer for three years. It's a big job, and potentially dangerous for all concerned. Trapped birds can develop a potentially fatal muscle ailment called capture myopathy. Bird-banders can get pecked or scratched.

The birds are caught with a rope snare that lies on the ground and is attached to a bent fishing rod. When a walking crane trips it, the rod recoils and tightens the leg noose in a way designed not to traumatize the bird. Two whoopers were outfitted with GPS anklets last winter. Start to finish, the procedure took 15 minutes for one and 16 minutes for the other.

Each device costs $4,500, and the tracking service, provided by a private company, costs $1,500 a year. The anklets are solar-powered and designed to last at least three years. In a trial run the researchers last spring put devices on two sandhill cranes; they're still working.

With a weight of less than three ounces -- a little more than 1 percent of an adult crane's weight -- the tracking device isn't expected to hamper a bird's flight. Every six hours it records its position and uploads the information to a satellite. If a crane's location doesn't change for 24 hours, the scientists monitoring the data stream will conclude something has happened and go looking for the bird.

More than 90 percent of the scheduled transmissions from the two whooping cranes have come through. Late last week the birds were in Kansas, not Nebraska; near each other, but apparently not traveling together, Gil said.

What kills whooping cranes is only the crudest of facts the scientists hope to get out of the project.

"We hope to learn something about their habitat-use patterns," Chavez-Ramirez said. "Where do they spend the night? What are the characteristics of those sites, the depth of the water, the vegetation? We've never had a quantifiable way to evaluate where they roost."

Wetlands are being lost throughout the birds' migration route, he said. The tracking program "will let us understand what they want and what they use so that perhaps we can reproduce those conditions."

It may also help answer more speculative questions.

As part of her research, Gil has correlated climatic variables with the whooping crane population size and the reproductive success of birds identified by colored leg bands (including one four-generation lineage). She has found that the population declines in rough synchrony with the Pacific Decadal Oscillation, a pattern of Pacific climate variability similar to El NiƱo. This decline is probably due to weather extremes during those years: low temperatures, which stress the birds, and a shortage of rain, which lowers water levels and exposes their nests to predation.

A better picture of where the birds are, and how they die, will help test her hypothesis.

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