Sooty Tern Migration

In Florida’s Dry Tortugas, these seabirds are nesting three months earlier than they did in the 1960s; scientists trying to unravel the cause suspect global warming is a factor

12-01-2007 // Michael Tennesen

ON A WARM SPRING afternoon, biologist Fernando Colchero wanders through a colony of sooty terns on Florida’s Bush Key, a sandbar perched atop a coral reef surrounded by clear tropical waters in Dry Tortugas National Park. Lying 70 miles west of Key West, the Dry Tortugas form a cluster of seven waterless islands famous for their bird life and historical military fort, which dates to the 1840s. Bush Key, part of the group, measures only 100 to 130 feet wide by half a mile long, yet 50,000 to 75,000 breeding adult sooty terns come to it each year—the species’ largest U.S. nesting colony and one of the biggest in the Caribbean.

As Colchero walks, he counts the number of nests and the amount of vegetation at 130 points across the island, which he visits twice yearly. All around him the birds shriek. Sooty terns are called “wide awakes” not only because one of their calls sounds like wide-a-wake but also because of the volume. “It never stops,” says Colchero, a doctoral student at Duke University who has been studying the birds for five years. Betty Anne Schreiber, a research associate for the Smithsonian Institution’s National Museum of Natural History, once took a decibel meter into a Pacific colony of sooty terns and found that the reading was so high, “It was against OSHA standards to expose workers to that level of noise for more than 30 minutes.”

Colchero is investigating how tern population dynamics are affected by environmental factors such as climate. His work is based on studies begun in 1959 by Bill Robertson, a former biologist with the National Biological Service of the U.S. Geological Survey, and his wife Betty, who collected data on the island until 1980. They spent 40 years analyzing their information but passed away before publishing their results.

By studying the Robertson data, Colchero says, he has found that the terns are beginning their breeding season earlier than they did in the past. In the 1960s, breeding terns began arriving on Bush Key in April, but now they begin nesting in late January. According to Colchero, this change is “the most dramatic shift in the nesting season for any bird species.” The new schedule is doubly interesting, he adds, because “normally you find such shifts in birds that live in temperate areas, but these birds are tropical. They don’t migrate long distances.”

Sooty terns are the most abundant seabird in the Tropics, numbering 60 million to 80 million. Elegant creatures weighing only 7 ounces, with an average body length of about 16 inches and a wingspan of 34 inches, they are black on top and white underneath with black and white markings around the face. They may be unique in the persistence with which they take to the skies. Not only are they so light and aerodynamic that flying requires a low expenditure of energy, but they apparently do not need deep sleep. “Short naps of a second or two in flight may be all the rest that is necessary,” Schreiber says.

Incredibly, this physiological peculiarity allows sooty terns to stay airborne for years at a time: Between fledging and first breeding, a period of four or five years, they never visit land. Once fully mature, they come to earth only to nest. During the non-breeding season they remain at sea, where they are never seen resting on water. “If they set down on the water for more than a few moments, they’ll become waterlogged and won’t be able to get up and fly away,” says Sonny Bass, the supervisory wildlife biologist for Dry Tortugas National Park. They feed by skimming over the water, using their beaks to scoop up squid or small fish.

At the beginning of the nesting season, the terns gather by the thousands over the Dry Tortugas in massive groups that resemble churning thunderheads. The birds may touch down on land for courtship ceremonies at night, but they don’t come to land by day until egg laying begins. They do not build nests but simply hollow out small scrapes in the sand in which they lay single eggs. Hatching takes about 30 days, fledging another 60.

Usually, sooty terns prefer to nest on bare ground with no vegetative cover, but the shift in nesting has changed that dynamic, Colchero says. The Dry Tortugas serve as a wintering ground for herring gulls, peregrine falcons and other predatory birds that feed on terns and, especially, their young. Before the shift in the nesting season, by the time the terns arrived in spring these hunters were largely gone. Now, terns and predators overlap more, a change that has made low vegetative cover extremely important for nesting terns.

Why they are arriving to lay eggs on Bush Key three months earlier than they did three or four decades ago remains a mystery that Colchero is trying to solve. Commercial fishing in the Gulf is one possibility. But Colchero does not believe that existing fisheries alone could have such a dramatic effect on fish availability. Global warming, however, may be the culprit.

The sooty terns of the Dry Tortugas are the only sooty terns in the Caribbean known to have shifted their breeding season, Colchero says. They also are uniquely situated in particularly warm waters known as the Loop Current, an ocean stream that flows northward between Cuba and the Yucatan, moves into the Gulf of Mexico and then loops around the Dry Tortugas before pouring into the Atlantic through the Florida straits to become the Gulf Stream. Based on preliminary data, the Gulf of Mexico shows no signs of a warming trend, Colchero says, but the Loop Current “is getting warmer, and this could affect productivity near the Dry Tortugas.” Katrina was diminishing in strength until it rode up over the Loop Current, which fed the storm’s intensity. Colchero is looking to see if similar dynamics of heat, as well as the direction and intensity of the current, might be affecting the transport of juvenile fish from the Gulf to the Dry Tortugas, which in turn might be pushing back the breeding season.

“We cannot say without a doubt that the earlier nesting of these terns is due to global warming, but the patterns we’ve seen make global warming the prime candidate,” Colchero says. “Local upwellings, which bring up food for the birds, decrease with heat, and that’s one of the things we’re trying to find out more about.”

By nesting on remote islands and spending much of their time in the air, sooty terns have kept for the most part beyond reach of civilization. “But with climate change driving changes in the surface temperature, the intensity and frequency of hurricanes, and creating problems with fisheries, sooty terns may no longer be exempt from humanity’s influence,” Colchero says.

Michael Tennesen worked with Fernando Colchero while serving as a media fellow at Duke University.

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Hurricanes and Sooty Tern Survival

Sooty terns nest mainly on islands that may lie only a foot above sea level, where the vegetation the birds use for cover from predators is vulnerable to the elements. When Hurricane Katrina and several other major storms blew through Florida’s Dry Tortugas, home to the species’ largest U.S. nesting colony and one of the biggest in the Caribbean, in 2005, the storms removed 70 percent of the low-lying vegetation on Bush Key, where 50,000 to 75,000 breeding sooty terns nest yearly.

The following year the terns arrived, but few stayed. Some moved to nearby Long Key, but nesting was minimal. In 2007, they returned in force to Bush Key, but, after nesting, the majority of the birds abandoned their eggs and young. Betty Anne Schreiber, a research associate for the Smithsonian Institution’s National Museum of Natural History, says that such catastrophes are not unusual for birds that inhabit low-lying islands. “Sooty terns live up to 30 or 40 years, so adults can miss a season and still have plenty of time to replace their numbers in future years.”—Roger Di Silvestro

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