Secrets of the Seabirds

The birds are the first thing many visitors notice when they arrive at Dry Tortugas National Park. Clouds of sleek brown noddies wheel and screech between the keys, massive brown pelicans plunge into the shallow water after fish, and magnificent frigatebirds, prehistoric and alien with their enormous wings and long forked tails, drift ominously overhead.

In the middle of it all, forming unruly flocks thousands of birds strong, are handsome black-and-white seabirds called sooty terns. Though most species of tern — closely related to gulls — frequent beaches and marshes, sooty terns are at home only on the open seas, but starting in the winter, they return from the far pelagic reaches to the Dry Tortugas. It is their only nesting colony in the United States.

More than 500,000 sooty terns have been banded in the Dry Tortugas since the 1950s, and the Park Service follows them closely from the moment the birds arrive in mid-January until they begin leaving in July. Staff have an exceptionally detailed understanding of the birds’ breeding habits and activities on the islands but have known almost nothing about the rest of their lives. Where do they go when they aren’t sitting on their nests, and what route do they take from the Tortugas? More important, what kind of dangers might they be facing out there on the open seas, and can that information be used to learn about the health of the ocean?

Those were some of the questions on Ryan Huang’s mind when he brought tiny bird backpacks containing solar-powered geo-transmitters to the Dry Tortugas in 2014. “It’s impossible to protect a species if you don’t know where it is,” said Huang, a graduate student at Duke University. The backpacks transmitted coordinates to Huang’s laptop, so he could track the birds’ daily activity from his apartment in North Carolina.

“It was a lot of fun. I got to sit down each morning before work and see where the birds were going,” he said. Thanks to the telemetry data from those birds, combined with sightings of banded Tortugas birds in other locations, Huang has started to piece together a picture of the routes sooty terns take to their wintering area.

As it turns out, after leaving the Tortugas in July, sooty terns spend about a month feeding in the Gulf of Mexico before beginning to fly south out of the Caribbean Sea. They follow the eastern coast of South America, then head out to sea, spending the rest of the winter above the open ocean between Brazil and Africa.

Once he was armed with a map of sooty tern migration, Huang could determine where the birds might run into trouble. One threat was immediately clear: hurricanes.

Hurricanes and tropical storms are a regular occurrence on the Dry Tortugas, and the Park Service has long understood the dangers that strong storms pose to nesting seabirds: After a direct hit from a hurricane, the local population can experience a temporary dip in numbers. Huang was surprised, however, to learn about the dangers hurricanes pose to migrating sooty terns. Combining the telemetric data with historical reports, he found a strong correlation between sooty terns’ migration patterns between the Tortugas and South America and bird deaths associated with hurricanes.

“Migration itself is a very stressful, very taxing process for these birds to undergo,” said Huang. “Encountering a storm adds even more stress, forcing the birds to fight strong winds and rain. Those that can’t handle that will likely die.” Unlike many shorebirds, sooty terns do not have special feather oils to help repel water, so they’re susceptible to drowning.

Whereas their terrestrial counterparts can ride out strong storms in dense foliage, seabirds have nowhere to hide from a hurricane. If birds are lucky enough to make it into the relative calm of the storm’s eye, they may stay in that pocket until the storm dissipates, often leaving them hundreds of miles off course. In the wake of Hurricane Irma in September, sooty terns, as well as other seabirds including black-capped petrels and brown noddies, were found as far inland as Tennessee and western North Carolina.

Climate change, which scientists expect to foster stronger and more frequent tropical storms, could lead to more sooty tern deaths. The birds are generally south of the West Indies by September, meaning late-season hurricanes like Hurricane Irma don’t pose a great threat, but summer hurricanes could be dangerous for southbound sooties. “Whether or not birds get caught in a storm during migration is all about the combination of time, intensity and location,” said Huang. “And as the number of storms increases, the chance of those factors coming together also increases.”

Sea-level rise caused by climate change is also a major concern for the Dry Tortugas population, according to Park Manager Glenn Simpson. “These islands are low-lying and are sensitive to any changes in sea level,” he said. The sooty terns are important to the park — the species is specifically mentioned in the 1992 enabling legislation that established the site as a national park. “I was happy to go help put backpacks on those birds if it helped us protect them,” Simpson said.

Huang and his colleagues hope to continue their work tracking sooty terns because the species is a particularly useful indicator of ocean health. Though the Dry Tortugas are the terns’ only American nesting colony, the birds are common in tropical waters around the world. “Sooty terns are a great sentinel species because they have a big range, meaning they’re sensing a lot of the ocean for us,” Huang said. For example, studying seabird diets can help experts predict fishery declines much earlier than traditional measurements using catch data. Humans are good at removing every last fish in a particular spot and often see full nets right up until a drastic decline; seabirds figure out they need to supplement their diet sooner and start looking elsewhere.

Huang and other scientists who are studying sooty terns hope that they can help uncover other changes in the ocean. Watching the birds could alert researchers to an oil spill, overfishing or problems associated with rising and warming seas. “These birds are a lot more sensitive to changes than humans are,” he said, “and they’re helping give us an idea of what’s going on out there.”