By Pat Leonard

Red Knot on the nest, Alaska.  Photo by Benjamin Clock

How do you follow a flying bird for thousands of miles? Putting a radio collar with a beefy battery on a jaguar is one thing—making tags small enough for a bird to carry for a long period of time has been impossible for all but the largest species. Cornell Lab of Ornithology scientist Dr. Alejandro Purgue has accepted the challenge. He’s testing his custom-designed mini-tags and receivers on a migration superstar—the Red Knot. Purgue’s mission, funded by the National Science Foundation, is to create a tracking system that will allow scientists to collect information about birds all along their migration route—even if the round trip is more than 18,000 miles long.

Far-flung Field-tests

In April 2007, Purgue began field-testing his tracking system in Mauritania on the west coast of Africa. Mauritania was chosen because researchers from the Netherlands Institute for Oceanographic Research  have been working with this system for years, and they have research sites covering most of the Red Knot migration path.

Hazy sun in Mauritania's coastal desert. Photo by Alejandro Purgue

From the capital, Purgue and his colleagues traveled north to a government field station based at Iwik, accessible only by four-wheel drive vehicles, boats, or small planes. Next came a three-mile hike to the Banc d’Arguin Reserve. The area is chock full of expansive tidal mud flats that attract huge concentrations of Red Knots and other shore birds. According to the park’s web site, it hosts the largest concentration of wintering waders in the world. The knots spend several months there in the fall, fattening up on worms and mollusks for the long trip to their breeding grounds in north-central Siberia.

 “The migration is so taxing that they have to more than double their body weight before they can attempt it,” says Purgue. “Their normal weight is about 100 grams and before they migrate they weigh 220 grams.” The knots head north in mid-April, making stops along the way in France, the Netherlands, and Germany. (Red Knots in the Western Hemisphere travel between South America and the Arctic, with a major stopping point in Delaware Bay.)

To Tag or Knot to Tag

First step: tag some birds. Purgue and his colleagues set up a corridor of mist nests along the shore to capture knots as they followed a predictable route back to their roosts. “Generally, the recovery rate for tagged birds is about 50 percent so we wanted to tag at least 30 or 40 knots,” he says. “But the winds were very strong—gusting up to 70 miles-per-hour—so we were only able to capture nine birds.” For a formal migration study, that number would be too small to produce significant data, but the goal in this case was to test the equipment.

Each knot nabbed had the mini-tag glued to feathers between its shoulder blades. Each tag weighs about 1.2 grams and falls off when the birds molt. “Five minutes after they are attached,” Purgue says, “The bird preens its feathers over the tag and you can’t even see it.” Data on each bird’s age, sex, weight, length, etc. were also collected. Some Red Knots sported the new tags and others were outfitted with a commercially-available transmitter used for short-term tracking. The hope was that signals from these short-distance tags would be picked up at the birds’ rest stop in the Wadden Sea north of Germany to confirm their location. No signals were detected from these other tags, however, and they may have been lost.

The Return Trip

Alejandro finds a "shady" spot in the desert.

Purgue and the Red Knots both made the return trip to the Mauritania mud flats, right on cue, five months later. He and the Dutch team spent three weeks at the study site setting up a receiving station and checking to see if any of the returning knots carried the mini-tags with downloadable data about the birds’ journey.

Each tag carried an internal clock and was programmed to start looking for receiving stations in mid-August. During the long trip, the tags sent out a brief signal each hour at a specified time. Every day the tags recorded and stored time of sunrise, sunset, altitude, and temperature. The receiver, also designed by Purgue, contained a computer hard drive, memory, GPS unit, and radio transceiver in a waterproof, sand-proof case.

 “The receiver will try to talk with the tags and once they receive data they will store it in the permanent memory. Even if the batteries run out you can retrieve the data,” says Purgue. When the data is downloaded, scientists have an unprecedented day-by-day log of the bird’s travels.

Mixed Results

“From the start we said, this is completely experimental and there’s a very good chance that we won’t get any data back,” says Purgue. After the first trip in April he found a coding error in the tag. It was easily fixed, but by that time the knots were long gone and it meant the tags would have problems communicating their data. Another disappointment: none of the tagged birds were detected when the knots returned in August.

Setting up the antenna array, hoping to hear signals from the mini-tags.

The exercise also highlighted what worked. “It was extremely important to see what it was like to run a study of this type under such conditions,” says Purgue. “On a couple of the days the temperature was 120 degrees. There was constant wind and sand—deploying electronic equipment under those conditions is incredibly hard. The sand was a huge problem, but all the equipment on the receiving end worked very well—it survived the heat and the dust for over a month without any problem.”

The plan is to tag the Red Knots again in April 2008. Before then, tweaked mini-tags will be tested on captive birds in the Netherlands and on non-migrating birds in Mauritania. One improvement will be to the tag’s thin antenna. Although copper-coated stainless steel wire was used the first time, the antennas became heavily corroded when left outdoors for several months. Purgue will try other materials to see if they hold up better.
The coding problem has already been solved.

Sun, Sand, Sea…and the Future

It’s been a real adventure for Purgue, who had never been to Mauritania before. The study site sits between two extremes: the Atlantic Ocean and the Sahara Desert, a world defined by the horizontal lines of sea, sand, and blue, blue sky.

A colorful mobile home. Photo by Alejandro Purgue

 “The environmental conditions were quite difficult for scientific study,” he says. “In April we camped in a big desert tent, a huge thing—very pretty. But it was very windy and the problem was trying to sleep at night with the tent canvas always flapping. There was very fine dust everywhere and after a couple of nights even breathing was a problem.”

As for the Red Knots, their future is uncertain. Studies of this chubby sandpiper have revealed drastically declining numbers. For example, between 1980 and 2000, the number of Red Knots surveyed in South America has dropped by half. Scientists speculate that because the knots congregate in such large groups, any contaminant, reduction in food supply, or other threat affects all of them at once.

Purgue will persevere with his tracking system, which has the potential to reveal so much about where migrating birds travel, what they need along the way to survive the journey, and the impact of human activities on sensitive habitat.