As winter approaches, many seabirds that nest along Atlantic coastlines in summer head out to sea to live and hunt in cold, northern waters. Dovekies, one of the most numerous North Atlantic seabirds, dive deep below the waves to feed on tiny crustaceans. Atlantic Puffins forage for fish and mollusks. Black-legged Kittiwakes eat just about everything, from pollock to squid to marine worms, scooping prey up from the surface. Millions of these and other seabirds winter in the North Atlantic—but where exactly they go, and what threats they face while there, are long-standing mysteries of seabird science.
“We often know very little about where seabirds actually are during the winter,” says Don Lyons, director of the National Audubon Society’s Seabird Institute in Maine.
Researchers now have some new answers thanks to recent analysis of bird-tracking data compiled by scientists around the world. One recent study, published in August in the Journal of Climate, identifies a 220,000-square-mile "hotspot" southeast of Greenland where millions of marine birds spend the winter. But the excitement of the discovery was quickly dampened by a second report, published last week in Current Biology by a different group of scientists, which describes in detail how wintertime storms starve birds and contribute to mass die-offs called "winter wrecks." As the authors of the paper point out, according to some climate models, in the future winter cyclones could intensify as seabirds continue to concentrate in this same hotspot region—a possibility that has experts concerned.
“The interplay between these two studies is worrying,” says Iain Stenhouse, director of the marine bird program at the Biodiversity Research Institute in Portland, Maine, and a contributing author on the hotspot study. “We just identified this area that that hosts millions of seabirds, and if you could imagine one of these really large-scale storm events ripping through that area at just the wrong time, you could have a really big problem. One event like that has the potential to impact seabirds across an enormous geographical area.“
In the first study, researchers pulled together more than 2,000 seabird movement records from the non-breeding season collected from the BirdLife Seabird Tracking Database. They estimate that each winter between 2.9 and 5 million birds from 21 species converge in a patch of ocean about the size of France located in the waters just south of Greenland and Iceland.
The second study looked into seabird vulnerability in the face of severe weather, focusing on midlatitude cyclones—swirling storms with heavy winds and rain that form over the ocean between North America and Europe, mostly in winter. The researchers focused on five species affected by these cyclones: Dovekies, Atlantic Puffins, Common Murres, Thick-billed Murres, and Black-legged Kittiwakes, which together make up more than three-quarters of all North Atlantic seabirds. Using tracking information for more than 1,500 individual birds, weather data, and computer models, the researchers found that winter cyclones kill birds through starvation. During and after intense storms, birds can’t access food either because rough seas prevent them from diving or because their prey relocates.
These winter cyclones are a common cause of seabird wrecks, which occur on both the North American and European coasts fringing the Atlantic. During a wreck, beachcombers can find dozens to thousands of dead seabirds washed up onshore. The devastation can span entire coastlines, depositing death across miles of beach. Often, the deceased birds are emaciated because of starvation and sometimes, they’re not yet dead but in the process of dying. “You’re seeing birds that should be at sea, and really at home on the water, struggling on land,” Stenhouse says. “That can be quite depressing: to watch birds dying and feel that you’re unable to do anything for them.”
The birds that make it to shore are a fraction of a storm’s full mortality, says Lyons. Currents can carry dead birds away from land, where they’ll never be discovered by scientists or beachcombers, or they can sink into deeper waters. Because of this, it’s hard to know the full impact of cyclones and wrecks on seabird populations—but winter is when seabird populations experience their biggest losses overall. “How much of that is due to winter storms versus other threats?" Lyons says. "It’s hard to gauge, but I think winter storms definitely deserve more attention.”
Climate change could exacerbate the impact of these winter cyclones and the magnitude of future wrecks. Some models and climate scientists predict that the frequency of the strongest North Atlantic winter storms may worsen in a specific ocean patch southeast of Greenland—an area that heavily overlaps with the newly identified seabird hotspot. In this region, “any given storm in the future is expected to produce more precipitation and be accompanied by stronger winds,” wrote Allison Michaelis, a Northern Illinois University climate scientist and lead researcher in a 2017 study on the subject, in an email to Audubon.
Seabirds already face multiple threats to their survival, making them one of the most imperiled bird groups. Oil and plastic pollution, habitat loss, commercial fishing, climate change-related food availability, and invasive predators at nesting sites are just a few of the ways that humans have put marine bird populations at ever-increasing risk, says Sarah Gutowsky, an ornithologist at Dalhousie University in Halifax, Canada. The devastation of winter wrecks cause by cyclones is “the cherry on the top of the cake,” she says. Stenhouse agrees. He describes seabirds as surviving in a narrow margin. “Any increase in mortality or unpredictable events like storms is going to have a big impact on populations.”
Though concerning, the pair of studies can also help scientists prioritize seabird conservation efforts. While the results are yet another argument for trying to minimize climate change, some change is inevitable based on current carbon dioxide levels. So conservationists should approach seabird protections holistically, experts say—by, for instance, bolstering protections for seabird breeding colonies and better preserving the birds' food supply through fisheries policy. “It's more reason also to take the other threats even more seriously and address them,” Gutowsky says. “The more you can do to reduce the struggle for these birds, perhaps the more they could weather the storm from the impacts of cyclones.”