Secrets of Willet Migration Revealed

Olympic athletes are impressive, no doubt. But when it comes to athletic prowess, few creatures compare to migratory birds like the eastern willet.

A large shorebird with distinctive white racing stripes and a unique penchant for nesting in salt marshes, the willet flies at speeds up to 57 MPH to cross the Atlantic Ocean in just 3 days. Leaving its nesting grounds at the Delaware Bayshores by early August, willets cover some 3,500 miles before eventually settling down for the winter in … where?

The secret location of willets’ wintering grounds had been a mystery—until now. Conservancy ecologist Joe Smith studied willet migration for the past 3 years and discovered that the eastern willet winters in estuaries on the northern coast of South America, mostly in Brazil and Suriname.

His discovery comes with the aid of new geolocator tags that rely on hours of sunlight to pinpoint latitude and longitude. Geolocators are lightweight, small and cheap—allowing scientists to track more types of birds, like willets, that were considered too small for the bulkier satellite tags traditionally used.

Such advances in technology are ushering in a new era of bird studies and giving scientists deeper insight into the full annual life cycle of migratory animals, a burgeoning field known as “migratory connectivity.” It goes beyond discovering where birds like willets winter to answering more complicated questions about how they migrate—such as how many miles they travel per day and how many places they stop. Migratory connectivity even uses advanced analysis of birds’ chemical makeup to understand what the habitat conditions are like at their wintering grounds.

“It’s a golden age for tracking migration,” enthuses Smith. “We’re learning that it’s a big world, yet it’s a small world for these birds because they use very discrete places.”

And these advances in science can mean better conservation for birds like the willet.

(See a photo slideshow of Smith catching willets as part of his research.)

An Overlooked Species of Scientific Research

Very few studies have been conducted on eastern willets, with only one significant research project in the 1970s. Smith’s study will provide the richest collection of data about the migration of this species to date.

In addition to the geolocator tags, Smith’s team collects feather and blood samples that are then analyzed for mercury and carbon and nitrogen isotopes by experts at the Biodiversity Research Institute (BRI) and Smithsonian Migratory Bird Center. Together they’re piecing together a more complete picture of the annual life cycle of an eastern willet.

The birds’ primary wintering area in Brazil happens to be a shorebird hotspot—it has the largest tract of intact mangrove forest in the Western Hemisphere and is sparsely populated. It’s not a big surprise that the willets spend their winter here, where the mangroves are lush and the food is plentiful.

What is a surprise is the other discovery that Smith’s team has made: willets are picking up mercury pollution at potentially harmful levels at these seemingly pristine wintering grounds.

“We know this because of the feather samples,” explains Smith. “Feathers retain a signature of the habitat conditions where they were grown. When the birds arrive here to nest, we can get a really good sense of what their winter habitat conditions were like from feather samples.”

Mercury & Migration Don’t Mix

The cause of mercury pollution at such a pristine place could be nearby gold mining, a well-documented source of methylmercury in the Brazilian Amazon (methylmercury is the organic form of mercury that can be harmful to people and wildlife). It’s also possible that the source-point is further away, since mercury released into the atmosphere can travel long distances before being deposited in an ecosystem and undergoing methylation.

But what the mercury means for willets is still unclear.

“We know that mercury in loons creates asymmetrical wings, so we’re looking out for that in our birds,” says Smith.

Wing asymmetry spells trouble for long-distance migrators like willets, says Dave Evers of the Biodiversity Research Institute. He’s been researching the effects of mercury on wildlife for years and first discovered the problem of wing asymmetry in loons. More recently, he’s documented mercury’s impact on insect-eating songbirds.

“If you have perfect symmetry, you’ll have an easier time flying,” he explains. “Controlled studies of starlings in wind tunnels have shown that a 5% difference in wing symmetry can cause a 20% increase in energy output during flight.”

Shorebirds could be particularly vulnerable to mercury for two reasons: 1) wet areas, such as the estuaries where willets live year-round, typically exhibit the highest methylation; and 2) mercury is stored in the muscles—when migrating birds burn through their fat stores, they then rely on muscle energy that could release more mercury into their systems.

“My concern is for birds like willets that migrate long distances and build up mercury in their bodies without having time to get rid of it,” says Evers.

Rising Seas, Raising Questions

Smith’s 3-year study is just concluding—next there will be many months of analyzing and interpreting the rich storehouse of data collected. Further analysis will reveal if the willets exhibit asymmetrical wings or high levels of mercury.

And there will be other mysteries to explore, including the million-dollar climate change question: what will happen as rising sea levels transform the estuarine habitat that willets rely on?

“Willets are the only shorebird to nest in salt marshes,” says Smith. “Given that all the habitat they use is only inches above the current sea level, even the most conservative predictions of sea-level rise will affect them.”

Another concern is habitat destruction from human development. Shrimp farming is growing rapidly in Brazil, taking down huge chunks of mangroves with it.

“Shrimp farming isn’t prevalent here yet, but this very pristine place could be threatened in the future,” says Smith. “Shrimp farming seems to be creeping westward along the coast and Brazil recently passed a law that makes it easier to develop protected areas.”

Smith is optimistic that the questions can be answered and solutions found. He sees a silver lining in the fact that the birds’ wintering area in Brazil is mostly populated by subsistence communities.

“Activities like industrial shrimp farming displace people who live subsistence lifestyles, as well as the wildlife and the ecosystems,” he explains. “The birds don’t have a voice, but the people do. My hope is that the people living here can be allies for conservation in the area.”

(Image: An eastern willet in flight. Source: Brian Hart 2011.)

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  1. Good discover!

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