Watch the skies and learn where the mercury lies.

Arctic seabirds called little auks (Alle alle) pick up mercury while on holiday in southern climes, a new study reveals, and then subsequently transport the toxin back into their main habitat. By tracking the pilgrims, scientists can pinpoint oceanic pools of the pollutant and possible sites of food chain contamination.

Pumped into the air by coal-burning factories, mercury has dusted the Arctic (and the rest of the planet) since the industrial revolution. In 2011 the Arctic Council, an international forum that governs the geopolitical issues of the north pole, published a report concluding that mercury exposure had increased 10-fold for some arctic species, including beluga whales and polar bears. Perplexingly, other creatures, such as walruses, had not absorbed the neurotoxin at all.

The scenario for seabirds was murky, too. The council’s 2011 report suggested that arctic avian migrants have gotten a break from mercury exposure by traveling to lower latitudes in the North Atlantic compared with nonmigrating species. “Our new study demonstrates that this is not always the case,” says co-author Jerôme Fort, an arctic ecologist at the University of La Rochelle in France.

Little auks (pronounced “ox”) dominate the Arctic. Up to 80 million stretch across the frozen wilderness from Russia to Iceland, where they live from February to August. When hungry, the coastal birds cruise seaward to a floating ice sheet. They dive into the cold water, like penguins, in search of tiny crustaceans. When autumn arrives, the little auks seek warmer climes, south of the Arctic circle, such as the coasts of the U.K. or Canada’s eastern shores.

Each summer since 2005 the French Polar Institute has sent Fort and his fellow ecologists on an Arctic voyage to investigate the impacts of ongoing environmental changes on seabirds. They began to ponder if mercury exposure varied by season in migrating seabirds. In July 2010 and 2011 the team traveled to East Greenland to visit the little auk colony in Kap Høegh, where they collected bird feathers for analysis.

Auk feathers from different parts of the body molt and regrow at different times of the year. Plumage from the head recycles in early spring, before the auks return to the Arctic; body feathers get replaced in October. Ocean mercury accumulates in the animals that the birds eat and then gets incorporated into the birds’ growing feathers, making each a time capsule of toxin exposure. Head feathers record mercury from the winter abroad whereas belly or back feathers do the same for the summer season at home.

The researchers found that the little auks’ mercury levels were 3.5 times higher during their winter furlough versus when the birds lived in the Arctic.

Fort and his team had also given leg bracelets to 135 little auks before their southern voyage. Each one contained a miniaturized computer that calculated the birds’ geographic location by intermittently recording light levels and the position of the sun. The migrants traveled to Canada’s east coast and mercury exposure varied according to their destinations there (see map). The researchers joined one group of little auks near Newfoundland and took blood samples from untagged birds. When compared with similar specimens from East Greenland, the team again noted 3.5 times more mercury exposure in the birds when they were vacationing compared with when they were in residence in the Arctic.

These seabirds could be used to identify environmental pollution on a large spatial scale, Fort says. The scientists suspect that when they journey to the Canadian coast the little auks switch their diets to fish, which more readily accumulate mercury than the birds’ normal fare of crustaceans. The birds dining fare may thus reveal tainted fish populations.

In Greenland Arctic foxes and glaucous gulls prey on little auks, and polar bears poach the tiny birds’ eggs, suggesting that the mercury acquired by little auks could spread further up the food chain.

How the mercury affects the birds’ overall health is not yet known. But Fort notes the team found higher feather toxin levels in female auks correlated with smaller eggs even though breeding occurred months after the birds returned from their sojourns.

As far as researchers know, little auk numbers are not decreasing but no one has monitored the animals like this before, says ecotoxicologist Geir Wing Gabrielsen of the Norwegian Polar Institute. Using geotrackers to measure seasonal changes in contaminant exposure in seabirds only became popular three to four years ago, he explains. In 2013 Gabrielsen and his colleagues reported that the great skua, another Arctic seabird, transports organic pollutants from eastern Canadian waters to Iceland. Such studies provide really good insights for environment toxicology, and may help explain why certain migrating arctic seabirds such as kittiwakes and thick-billed murres are in decline, Gabrielsen remarks.