Whooping cranes will be tracked with GPS units to study their habits and hazards
Tuesday, March 30, 2010
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.