Another bird with similar equipment detoured 620 miles to avoid a tropical storm.
"It's almost unthinkable what these birds are doing," said Charles Duncan, a scientist with the Manomet Center for Conservation Sciences in Maine, who was not part of the study. "I go for a two-mile run and think I'm a big athlete."
The recently published research, led by Conserve Wildlife Foundation of New Jersey biologist Larry Niles, is being praised not only for its scientific value.
It also is seen as a breakthrough for avian tracking equipment, which has borrowed from the development of ever-shrinking consumer-electronics devices.
The journey of the red knot had been sketched out by Niles and other scientists, based on tagging a bird in one place and sighting it again in other places.
They have long known that many of the birds stop each spring at Delaware Bay. Emaciated from such long flights, they need to refuel on the lipid-rich eggs of the horseshoe crab, which is just then swarming ashore to spawn.
But the new devices have confirmed both the distance and each bird's route.
For Niles, who has trailed the bird nearly to the ends of the earth, camping on Arctic rubble and slogging across Tierra del Fuego's windswept mudflats, the findings were a personal revelation.
Before, he had focused on the vulnerability of the overall red knot population, which is a "candidate" for protection under the federal Endangered Species Act.
Now, Niles was riveted by the fragility of a single bird.
"Imagine a bird flying for six days over the Amazon, where there is no suitable habitat for it to forage, and then over the ocean, where it can't land," he said.
"It's burning off every ounce of strength it has, and then it gets to Delaware Bay, and the crab eggs are gone."
Researchers blame the bird's decline on overharvesting of horseshoe crabs. The crabs become bait for conch, sold in Asia as a delicacy.
The research was funded by the U.S. Bureau of Ocean Energy Management, Regulation and Enforcement, which oversees offshore energy projects, such as wind farms.
The bureau wanted to get better data for potential effects on the red knot and two other species - the endangered roseate tern and the threatened piping plover. Both birds had sparked controversy for the Cape Wind project off Nantucket.
Niles and an international team of shorebird researchers that converges on the Delaware Bay every spring put the devices, called geolocators, on 47 of the red knots they captured in May 2009.
It would be a year before the scientists would know for sure where the birds went.
Basically, the geolocators determine location by measuring daylight. The length of a day shows the bird's latitude, and the time of local "noon" - when the sun is at its highest - shows longitude.
These tiny data loggers are revolutionizing the science of tracking birds.
They are quite an improvement over just 10 years ago, when Niles was the first to zero in on the red knots' Arctic nesting territory. He did it by gluing VHF transmitters to the birds' backs.
But the signal was audible for only about eight miles. So up in the Arctic, Niles chartered a plane and they swept the region in a grid pattern, listening for a telltale signal. Incredibly, he heard some.
Far better devices exist than even the geolocators, but only for larger birds. Several years ago, Pennsylvania officials outfitted a peregrine falcon with a solar-powered backpack that transmitted fixed GPS locations to a land-based receiver - allowing scientists to fly along virtually via a laptop.
The geolocators were developed by the British Antarctic Survey in England. Every year, they are able to tweak the technology to get a smaller instrument, said James Fox, an electronics engineer with the Survey.
The problem is that the devices for the red knots need to be so small that it's impossible to add the capability to transmit. To get the data, the scientists have to find the birds and recapture them.
Fortunately, Delaware Bay is an ideal spot because so many red knots stop there, and Niles and his crew have become experts.
First, they note where the birds congregate at a particular time of day. Arriving ahead of time, they lay a long net in a shallow trench, covered with seaweed and attached to weights in buried pipes rigged like cannons.
Then, the researchers hide nearby, waiting for birds.
In this case, the scientists also had to do some fancy herding of particular geolocator birds - unsettling them enough to move them closer to the net, but not so much that they would fly away.
When they think they've got a catch, the researchers fire the cannon and the weights carry the net over the birds.
In May, the crew captured three with geolocators.
Some scientists had been skeptical. Would the devices be accurate enough in such a small incarnation?
"Now that the results are in, it's my new religion," Manomet's Duncan said.
Indeed, the results were so astonishing that even though Niles and his researchers had data from only three birds - eight more geolocators have since been recovered, and more than 200 additional red knots now have them - they decided to submit the results.
Their paper appears in the current bulletin of the International Wader Study Group, a shorebird research organization. Coauthors include Joanna Burger, a Rutgers University behavioral ecologist, and Amanda Dey, a biologist with the New Jersey Department of Environmental Protection's Endangered and Nongame Species Program.
The results help correct for a typical human bias - researchers study birds in areas where they find them. But they cannot know all the areas.
Two red knots stopped in one that the researchers never considered, the Lesser Antilles in the Caribbean.
The data also show increased importance for other sites once thought to be incidental - namely, New Jersey's Stone Harbor, which many red knots visit in the fall on the trip south.
Niles thinks that as they glean data from more birds, they may answer long-held questions about the red knot's survival strategy.
Some birds stop well short of southern South America, wintering near the mouth of the Amazon River. This saves energy, but the tropical Amazon has more avian diseases and parasites. How do the birds, in effect, choose?
Ron Porter, an Ambler engineer who designed tiny gizmos to attach the geolocators to the birds' legs, is still marveling at "the ability to know what the bird was doing (flying, sitting, feeding) each day for a year, around the world, and we only needed access to the bird in Delaware Bay. It's like getting back a note we sent off in a bottle!"
Contact staff writer Sandy Bauers at 215-854-5147 or email@example.com Visit her blog at http://go.philly.com/greenspace