Greater shearwaters live out their lives on the open ocean. These gull-like birds spend most of their time criss-crossing the Atlantic, covering thousands of kilometers on their annual migrations. But this year, many of them are dying along the way, and no one is quite sure why.
Greater shearwaters are one of the most abundant seabirds in the world. An estimated 10 million pairs nest on just a few small islands in the middle of the South Atlantic, about half-way between South Africa and Argentina.
Rob Ronconi, a Ph.D. candidate in biology at the University of Victoria in Canada, has been studying the birds. They weigh about one kilogram, he says, and their wingspan is just over a meter. He describes them as brown on top, and mostly white below, but with a good mix of brown and white all over their body, and a dark cap. "I tell my friends that they look like a small albatross."
Shearwaters spend three-quarters of the year out at sea, migrating in a giant figure-eight pattern across the Atlantic. Ronconi and his colleagues have used satellite tags to track the birds, from their northern foraging waters near Canada and Maine.
According to their observations, the shearwaters start their migration in early September. They cross the north Atlantic, flying through the Azores and down the coast of Portugal. "From Portugal they reach the coast of West Africa," Ronconi continues, and "from there they cross the Atlantic again." This takes them to Brazil, then south down the coast, where they spend the month of October foraging in the ocean off of Argentina.
The shearwaters then head over to their breeding islands in the South Atlantic. They remain there for about three months, each pair raising a single chick. In April or May, their migration begins again. The birds fly back across the Atlantic, and up along the eastern seaboard of the United States, to Canada.
Only this year, something went wrong.
Craig Watson is a wildlife biologist with the U.S. Fish & Wildlife Service. In the last week of June, reports started coming in to him from the east coast of Florida. "There was an e-mail that circulated around about folks starting to find dead seabirds, and these were mostly greater shearwaters," Watson recalls. He says he put out an alert to his contacts along the southeastern coast.
By his count, just over 2,000 dead birds have been found.
It's impossible to say how many shearwaters in all have actually died, because winds and currents would push only some fraction of the birds to shore.
The shearwaters have been tested for avian flu, toxic algae and other contaminants, but Watson says that so far, they have come up clean.
From all appearances, the birds are starving to death. But no one can say exactly why. "Some feel it's a natural phenomenon," Watson says, but he has doubts. "I think the thing that has caused some of us to question this is that it appears to be occurring more frequently."
There was a similar, but smaller die-off in 2005.
According to Rob Ronconi, it's not unusual for seabirds to have bad years. For young shearwaters in particular, getting enough food can be critical. The birds that have died this year appear to have been juveniles.
Ronconi says that they might not have gotten enough to eat before their long migration. "They need to gain a lot of weight."
They may have lacked food when they were being raised as chicks, he says, or just after they left the nest. He speculates that the timing of food availability could have been off, or that the young birds may have missed the feeding locations, en route.
Either way, says Ronconi, "if they started their migration underweight, they wouldn't make it all the way up to Canada."
Ronconi adds that there is still much to learn about shearwaters, if we really want to understand the causes, and importance, of this year's die-off. "Without knowing much about their basic ecology, their basic migration route, or even their basic weights," he says, "it's really hard to assess the impacts of these events, where you get mass strandings."
"These events could be part of the normal cycle," he explains, "or they could be unusual. At this point, we just don't know."
And knowing more about shearwaters, he adds, could tell us something about the health of the ocean, as well, "because seabirds, feeding mostly on fish and krill, can be a good indicator for other problems that are happening out in the ocean."
Ronconi will be studying the shearwaters as they spend the next several months feeding in the ocean water off Canada. He hopes his research can help biologists better understand what happened to the birds this year along the U.S. coast.