Sooty terns’ migration takes the birds into the path of hurricanes

Hurricane season has officially begun in the North Atlantic, and it’s not just coastal communities that have to worry. A population of sooty terns off the southwest tip of Florida might want to worry, too. Depending on when and where storms hit, the terns could be in for a tough time. Their migratory route overlaps with the general path of hurricanes traveling from the waters off Africa up to the United States, a new study finds.

Sooty terns can be found all over the world. But the ones that nest in the Dry Tortugas National Park, west of Key West, are among the best known. The birds have been the subject of a long-term study that started back in 1959, and of other studies that stretch back into the early 20th century. Those studies revealed much about the birds’ growth and behavior, but not much about the terns’ migration.

In 2011, Stuart Pimm, an ecologist at Duke University, and colleagues attached geolocators to 25 sooty terns. A geolocator is “a remarkably stupid device,” Pimm says. It simply records how bright it is every 12 minutes. From that information, researchers can determine sunrise and sunset from day to day — and therefore approximate the birds’ location. But they have to retrieve the devices to get the data. That meant finding those 25 birds in a population of 80,000.

The researchers managed to find two.

But those two birds had some remarkable data. The geolocators recorded that the birds had experienced a 12-hour day in December, offset by five hours from Florida. That meant that they had been flying somewhere around the equator and were headed toward Africa.

But perhaps those two birds were outliers. So in 2014, Pimm’s group got some more sophisticated technology that could transmit a bird’s location. The new tech was also a lot more expensive, so the scientists were able to track only five birds. But the researchers also didn’t have to wait a year to get the data — or search for the birds among a population of thousands. “I would get up every day and check on where the birds were,” Pimm recalls.

At least some of the terns were flying south through the Caribbean, southeast along the coast of South America and then to the middle of the Atlantic, where they spent the winter, the team reports May 10 in PeerJ.

It’s a path that takes the birds straight up hurricane alley the long way, Pimm notes.

The researchers then took advantage of all those decades of banding birds. They matched historical reports from 1960 to 1980 of wrecked (that is, dead) sooty terns with tropical storms and hurricanes. Some years the birds were fine, but, Pimm says, “some years they get absolutely slaughtered.” If a storm hits at the wrong place and the wrong time, the birds are out of luck. Even if they manage to survive the high winds and heavy rains, they can be blown far off course. Hurricane Camille, for instance, took one poor sooty tern to the Great Lakes in 1969.

In some years, hurricanes may take out a small portion of the sooty tern population, but it doesn’t appear to be enough to cause big declines. Pimm worries, though, about what might happen in the future. It is not yet clear how climate change might change the severity or frequency of hurricanes — and thus affect the terns — but it is something to keep an eye on, Pimm says.