Purple Sandpipers Feast on Barnacles along Maine’s Coast

From the Winter 2026 issue of Living Bird magazine. Subscribe now.

On Maine’s more than 4,000 offshore islands, winter brings biting winds, waves that crash into snow-draped rocks, and sudden storms that sweep in from the sea. Ice clings to shrubs and branches, and with the sun setting early, daylight hours are brief beneath a consistently overcast sky. It’s a landscape that challenges both people and wildlife.

And yet, flocks of one tiny shorebird can be seen here every winter, hopping around icy coves and salt-swept rocks in search of food.

“I have a lot of respect for Purple Sandpipers,” says Elliot Johnston, an ecologist at the Maine Natural History Observatory who has studied the state’s population of these birds since 2018. “They are a really hardy shorebird.” 

But Johnston and other scientists are noticing a marked decline among the Purple Sandpipers that winter along the Maine coast. Statewide surveys show the flocks are down more than 50% versus their numbers 20 years ago. A new study published in the journal Ornithology last July, and led by Johnston, may provide a clue to the decline—the wintertime diet of Purple Sandpipers in the state, long thought to be mostly blue mussels, now seems to consist mostly of barnacles.

According to Johnston, the study is a key to understanding the conservation outlook for these tough sandpipers that make their winter home farther north than any other shorebird species.

“It’s important to understand, if they are declining, what’s driving it,” says Johnston. “Diet could potentially play a role, especially for species wintering so far north.”

In the early 2000s, a crew of scientists from the Maine Department of Inland Fisheries and Wildlife and Acadia National Park conducted the first systematic survey of Maine’s coast to document the full scope of where these birds are found and to study their winter ecology. When the researchers repeated the surveys in the 2010s, in areas where they had found the highest densities of the birds, they found the population had dropped from about 4,700 Purple Sandpipers to an annual average of around 2,100 sandpipers.

Ecologist Glen Mittelhauser has seen the drop-off in Purple Sandpipers firsthand. He’s been conducting wintertime coastal bird counts for more than 40 years, starting with chartering rides out to sea with experienced Maine fishermen on days when the weather wasn’t too rough. Mittelhauser founded the Maine Natural History Observatory as a science-based nonprofit group to monitor changes in Maine’s plant and wildlife populations. These days, he sees far fewer of the diminutive, hardy sandpipers that he so admires—shimmering, he says, with a violet sheen when seen up close.

“These tiny little things … they’d be roosting in the sun all day, even though it was winter and wicked cold,” Mittelhauser says. “Now, it’s different. … I remember seeing a flock of 1,000 birds back in the ’80s and ’90s. Definitely don’t run into those anymore.”

The marine environments along the Maine coast have also changed a lot over the past few decades. According to the Gulf of Maine Research Institute, sea surface temperatures in the Gulf of Maine have been rising at an average rate of 0.84°F per decade between 1982 and 2024—nearly three times faster than the global average of the world’s oceans. That ocean-warming trend led Elliot Johnston, as a University of Maine PhD student starting his dissertation research eight years ago, to suspect there may be impacts on what Purple Sandpipers eat during the long winter months. 

The scientific consensus—based largely on research from Purple Sandpiper wintering areas in Europe—has been that these birds feed mainly on mollusks, including blue mussels, during the nonbreeding season. Marine surveys in the Gulf of Maine by ecologists from the University of California, Irvine, show that regional blue mussel populations are down more than 60% over the past 40 years.

“Between [ocean warming] and changes in ocean pH,” Johnston says, “mussels have declined.”

Johnston’s research project sought to use DNA metabarcoding—a relatively new method of analyzing what animals eat by sequencing traces of prey DNA left behind in feces—to learn more about the wintertime diet of Maine’s Purple Sandpipers. To gather the necessary specimens for analysis, Johnston and other graduate students suited up in full-body orange drysuits and mittens and set out for Maine’s islands during the winters of 2017–2018 and 2018–2019 to scoop up sandpiper poop. Once they spotted the birds, they’d watch for a while as the sandpipers hopped along the rocks. 

“As they were feeding, they were also defecating,” Johnston says. When the scientists figured enough samples had built up, they’d nudge the boat against a ledge, hop out, and start collecting. Equipped with sampling kits that included plastic cutlery, the researchers would scoop up the tiny droppings from seaweed or rock surfaces, preserve them in a buffer solution, and tuck them into a cooler for the trip back to the university. Later, the samples were sent to Northern Arizona University—home of the Pathogen and Microbiome Institute—to be analyzed. By the end of those winters, Johnston’s team had gathered nearly 200 samples.

Of the 24 prey types found through DNA analysis of Purple Sandpiper droppings, barnacles dominated—appearing in 88.5% of samples—while blue mussels, once considered a dietary staple, were scarcely present. Johnston says it is still unclear whether the birds are switching diets because warming waters have reduced mussel numbers, or whether the new method is simply seeing things that older techniques—such as stomach content analysis, in which scientists visually sift through what a bird has eaten—could not. 

Johnston adds that the decline in the Purple Sandpiper population is far from being definitively linked to their diet. Even if the sandpipers are switching from mussels to barnacles, that might be okay, he says.

“At the end of the day, they need calories to keep warm in the water and thermoregulate,” Johnston says. “If they came from barnacles, maybe it’s not a big deal.”

Jeff Foster, a coauthor on the study and professor in the Department of Biological Sciences at Northern Arizona University who specializes in DNA metabarcoding, says he had never used metabarcoding to examine a shorebird’s diet before.

“We had no idea if [this technique] would work for marine invertebrates,” Foster says, “so we thought we’d give it a shot. And it turns out, it works really well.”

Metabarcoding, Foster explains, works a bit like matching books in a library. Every species has its own sequence, like a book with a unique barcode. When extracting DNA from the sandpipers’ poop, researchers look for those books and try to match them to a global reference library of known species—things like mollusks, barnacles, or worms. When they find a match, they know what the bird was eating. 

Different Foods, or Different Methods?

Because the study results showed a completely different dominant food for Purple Sandpipers than previously documented, the journal reviewers asked Foster and his team to double- and triple-check their work.

The metabarcoding analyses, he says, “weren’t missing the mussels, and they were correctly identifying the barnacles.”

“It’s pretty certain that they’re shifting their diets,” he adds. “It takes a lot of time, and it takes a lot of technical expertise, but I really do think it’s a real shift in diet and not a methodological change.”

Other scientists say it’s hard to know if the birds are really changing their diet or if the new method is just detecting prey older studies missed. Gemma Clucas, a research associate at the Cornell Lab of Ornithology, uses similar methods to study seabird, songbird, and woodpecker diets. She says metabarcoding can be extremely sensitive.

“I do think it is a little bit difficult,” Clucas says, “to know whether what they observed is due to a shift in the diet of the sandpipers, or whether they’re just using different methods than previous studies have. … It’s potentially a mixture of both.” 

Clucas says metabarcoding is changing how much scientists can learn about bird diets. In the past, researchers were limited to labor-intensive data-collection methods, such as stomach content analysis or spending hours watching what parents feed their chicks. Fecal samples, by contrast, are easy to collect.

“With the terns I work on, they’ll just poop on you,” she says. “With the woodpeckers, we catch them, put them in a box, wait till they poop, and then let them go.” 

Collecting dozens of samples quickly gives scientists a far clearer picture of what birds are eating, she says. The technique has only been widely used for a little more than a decade, but it is already helping researchers understand why some bird populations are declining. 

And, Clucas says, up until now most metabarcoding studies have only focused on studying the diets of birds during the breeding season. This study, she says, opens up the conversation about a previously neglected part of the year—the nonbreeding season, which for many bird species is twice as long or longer than the breeding season. 

“We could definitely apply [metabarcoding] more broadly,” Clucas adds.

Brian Olsen, an ornithology professor at the University of Maine and coauthor of the study (and Johnston’s former advisor), said that if the results reflect a true shift in the Purple Sandpipers’ diets, it could help explain their decline in Maine and how they might be affected by future environmental changes. Purple Sandpipers are one of many species, he says, that inhabit a major ecological boundary between subarctic and temperate zones. 

“Those species in general are of conservation concern as things shift northward,” Olsen says.

What really struck Olsen about the resilient little Purple Sandpipers, he says, is how unconcerned the birds seemed while he was braving brutal winter conditions on bird surveys along the frigid Maine coast.

“You are living in a place that would make every single human being absolutely miserable,” Olsen recalls thinking about the sandpipers, “and you’re just as happy as could be.”