Paris (AFP)

Every winter, thousands of seabird carcasses wash up on American and European beaches in the Atlantic, in rather mysterious circumstances.

A study has just found the culprit: cyclones so violent that they prevent birds from feeding, and end up starving them.

Puffins, dwarfs and murres leave their nesting grounds in the Arctic every year to spend the winter further south, off Newfoundland, Iceland or Norway, in the middle of the North Atlantic.

They find milder conditions there: less freezing temperatures, abundance of food resources ... But all is not rosy there because in winter, these areas are on the path of high intensity cyclones that can last for several days.

"We suspected that these storms kill the birds. But what remained mysterious was to know where, and how", explains to AFP David Grémillet, CNRS research director who coordinated the study published Tuesday in Current Biology.

A puffin flies, its prey in the beak, off the Breton coast in June 2021 LOIC VENANCE AFP / Archives

To conduct the investigation, a large international research team decided to track five species from 39 colonies, representative of the North Atlantic bird community: Atlantic puffins, dwarf kittiwakes, and two species of murres.

They have equipped more than 1,500 individuals with GLS ("global location sensor") electronic geolocators, placed on the feet of birds in their various nesting sites in the summer, before they begin their winter migration.

- Ultra-light beacons-

Less precise than a GPS, these ultra-light tools require little battery because they simply measure the light to estimate the length of the day and deduce the latitude and longitude where the bird flies.

"The localization accuracy is of the order of 200 km", a scale sufficient to follow large movements of migrations, details the main author of the study, Manon Clairbaux, of the Center for functional and evolutionary ecology of the University of Montpellier.

A guillemot stranded in Soulac-sur-Mer, on the Atlantic coast in February 2014 NICOLAS TUCAT AFP / Archives

From one summer to the next, scientists manage to recover most of their beacons from birds returning to their nests, continues this CNRS researcher.

By studying their trajectories over ten years, combined with climatic data on winter depressions, the researchers were able to determine the areas in which birds encounter cyclones.

They then used a mathematical model that allows them to measure their energy expenditure in the face of weather conditions.

And discovered, to their surprise, that their energy expenditure did not increase with cyclones.

So if they do not die of cold, nor of exhaustion in the face of strong winds, "the assumption is that the weather conditions are so appalling that they cannot feed themselves", suggests David Grémillet.

- "Washing machine" -

"You have to imagine winds blowing up to 120 km / hour, waves of 8 meters, turbulence in the water column which disturb the plankton and the schools of fish on which they feed ... The birds are caught in a big washing machine ", describes this oceanographer.

Unable to fly far enough to flee, they find themselves trapped in the turmoil, doomed to wait for it to calm down.

And therefore probably prevented from diving into the sea to fish for their prey, or to see them in troubled waters ...

However, these birds with small wings have little fat reserves, and "a dwarf auk dies if it does not eat for 48 hours".

The carcasses littering the coasts, such as the tens of thousands of puffins and murres brought back by the waves in winter 2014 on French beaches, are moreover "particularly emaciated", adds Manon Clairbaux.

"It's important to understand the dangers that threaten seabirds," said the researcher.

Because their world population has already halved since the 1970s. Many factors explain this decline, such as accidental catches by boats, competition with fisheries, pollution on their breeding habitats ...

Cyclones are added to the list and worry all the more that according to climate experts from the UN (IPCC), "their frequency and intensity will increase with global warming", underlines David Grémillet.

Better mapping the areas where birds are subjected to them will make it possible to define upstream marine areas protected from all human activity.

© 2021 AFP