The Almost Impossible Mission of Studying the Lesser Yellowlegs

The most distinctive feature of the Lesser Yellowlegs (Tringa flavipes) is its bright yellow legs, which give it its common name. However, its most impressive characteristic is the ability to fly more than 4,000 kilometers in four days. Yes, that’s roughly a thousand kilometers a day, usually flying in flocks.

Now, let’s talk about another trait: being almost a master at evading mist nets. This has been confirmed, with much frustration, by Santiago Muñoz and Juan David García, two young biologists who have spent several migratory seasons setting up traps in rice fields across Valle del Cauca and Cauca—departments located in southwestern Colombia—to capture these birds. When they succeed, in a race against time, they must quickly remove the birds from the nets, weigh and measure them, check their overall condition, and attach a small (almost tiny, weighing 1.25 g—no more than 2% of the bird's weight) radio transmitter, before releasing them to continue their journey across the hemisphere. Considering all that’s involved, half an hour flies by.

But then comes the joy of a job well done. Santiago and Juan have spent entire days, sometimes before dawn, waiting for a bird to fall into the net. Many times (more than they would like to admit), they only catch dragonflies. So, after the adrenaline rush of attaching the radio transmitter to the bird’s back, they return home with the satisfaction of a mission accomplished.

And the joy of that day only grows when, thanks to the radio transmitter and the Motus Wildlife Tracking System, they can see that Cholao —the last tagged Tringa, named after a traditional dessert from the region where it was captured—weighing 113 grams (with an average weight between 65 and 115 grams), measuring 24 to 30 cm in length, and with a wingspan of 50 to 60 cm, has flown all the way to Texas, where it was detected by the tower located at Rockport Bay Education Center.

The path traced by Cholao’s transmitter appears as a straight line between Colombia and the United States, passing through Panama, Costa Rica, Nicaragua, Guatemala, and part of Mexico. If Cholao had been detected during that four-day period, or afterward, by another MOTUS tower—which has a detection range of 50 kilometers—it would have been possible to learn more about its journey: whether it made stops, its flight altitude, and other details. But it wasn’t. With no further detections, one might even believe it didn’t survive the journey.

Tringa flavipes is considered a vulnerable species by the International Union for Conservation of Nature, having lost 60–80% of its population in the last 50 years, according to the North American Bird Conservation Initiative. It's listed as a species of conservation concern in the United States by the U.S. Fish and Wildlife Service and is classified as threatened in Canada by the Committee on the Status of Endangered Wildlife in Canada.

It belongs to the family Scolopacidae, which includes other shorebirds. It is found primarily in North and Central America during the breeding season and migrates to South America for the winter. Individuals that breed in Alaska and Canada migrate to wintering areas in South American countries, reaching as far as Patagonia.

The Lesser Yellowlegs feeds mainly on insects, crustaceans, mollusks, and small fish, using its long, slender bill to probe in mud and sand. It prefers wet habitats such as swamps, marshes, beaches, and coastal areas during migration and breeding, which is why Santiago and Juan David hope to find them in rice fields—artificial wetlands that, depending on their growth stage, provide shelter and food for different bird species. However, these ecosystems can be affected by human development and pollution. The presence of birds like Cholao serves as an indicator of the health of aquatic ecosystems, underscoring the importance of conserving wetlands and coastal areas.

Why is this work in Valle del Cauca, Colombia, important? Over 80% of the wetlands that this species used have disappeared in the past 50 years in Valle del Cauca. Rice cultivation provides temporary habitat for the survival of the Lesser Yellowlegs in the region. Audubon is working with the sugarcane sector to promote crop rotation between sugarcane and rice, thereby providing habitat for this and other shorebird and waterbird species.

“In the first seasons, we worked from 5:00 a.m. to 5:00 p.m. We also tried 2:00 p.m. to 10:00 p.m., trying to figure out the best time to capture individuals, thinking they might not see the nets at night,” says Santiago, whose favorite bird is Semnornis ramphastinus, the Toucan Barbet—a bird iconic to northeastern Ecuador and western Colombia, which he first saw in a bird guide.

Now, also honing the art of waiting, the two biologists have reduced their working hours from 5:00 a.m. to 10:00 a.m. Their best day came in 2022, during the first season they spent two months trying to capture Tringas, in a joint effort with the Colombian NGO SELVA. In the first few days, they caught two individuals. Then, on the last day of the season, they caught a group of 11 individuals, eight of which were tagged with radio transmitters. Quite a feat!

That year, several individuals, including Cholao, made incredible flights to the U.S., according to data from MOTUS receiver stations. However, after reaching the U.S., the signals disappeared. These birds breed in the remote corners of the Boreal Forest in Canada and Alaska, a region rich in peatlands—a preferred nesting habitat for the Lesser Yellowlegs. What’s not abundant, however, are MOTUS towers.

The worst season for Santiago and Juan David came in 2023, when they failed to capture even one individual over four months of work, five days a week. “We don’t know what happened in the rice fields. We saw many large flocks passing through and were trying to refine our capture methodology. Although we did many tests, we achieved nothing, which was very frustrating,” explains Juan David, whose favorite bird is the Entomodestes coracinus, the Black Solitaire, because it was the first bird he ever captured using a mist net.

“When we don’t catch anything, we ask ourselves if we’re doing things right. So, we reached out to colleagues who do the same work, and they confirmed that the Lesser Yellowlegs is a very hard bird to catch. They’re elusive, have perfect vision, are very intelligent, and at the same time very calm. Others have tried different types of traps, always ensuring the bird’s well-being, and the results are similar,” says Santiago, explaining how difficult their mission is.

The challenge, in addition to the waiting, is placing the nets in areas with many individuals, as that increases the chances of capturing more birds. Fine-tuning their eye to find artificial wetlands, like rice fields, at their prime stage is truly a skill.

“Our work is the first step in describing the migratory routes of individuals of this species that pass through the geographic valley of the Cauca River, which has historically been a key migration corridor, with high wetland density and crucial stopover sites for shorebirds like the Lesser Yellowlegs,” adds Santiago.

Despite the effort—and the frustration of days without results—Santiago and Juan David say they love their work. It’s peaceful and requires a lot of birdwatching, which they’re truly passionate about. “There’s so much we’d like to learn, which is why we hope to tag more individuals and also achieve satellite tracking to gather much more detailed information about their routes and contribute valuable data to the conservation of migratory shorebirds that use these ecosystems. By knowing the sites, they visit to rest and feed, we can focus our conservation efforts on those places.”

Early Results and Learnings

Jorge Velásquez, Audubon's science director for Latin America and the Caribbean, explains that "although few transmitters have been deployed on Tringa, evidence shows that the tagged individuals remain in Valle del Cauca throughout their non-breeding period. This suggests that the population arriving in Valle del Cauca spends the boreal winter in the valley, rather than just stopping over on the way to other regions. Therefore, habitat management efforts in rice fields and wetlands in the valley will significantly impact the population that winters there. It has also been documented that the Valle del Cauca population uses the mid-continental route during its northward migration, while it uses both the Atlantic and mid-continental routes during its journey south. This has allowed Audubon’s conservation work in Valle del Cauca to connect with broader conservation efforts in North America, such as those at Audubon’s Riverlands Center and Corkscrew Swamp."