"This paper is perfect," said Martin Wikelski, a biologist at Princeton University in New Jersey who studies the migrations of songbirds and insects.
"It points the way this kind of research has to go in the future."
The study is part of a project called Tagging of Pacific Pelagics, which is tracking 23 top ocean animal species in the North Pacific to better understand their environment.
According to Shaffer and his colleagues, understanding sooty shearwater migration may help researchers monitor the health of ocean ecosystems and the impacts of climate change.
For example, the shearwaters' foodfish, squid, and krillare found in areas rich in microscopic plants called phytoplankton and other one-celled organisms that use sunlight to grow.
Such organisms form the base of the ocean food chain.
Studies suggest that climate change could cause areas where these organisms grow to shift or decrease in size.
"If climate change is affecting ocean productivity, and shearwaters are making long migrations to get to [these areas], it could have a large impact on the population," Shaffer said.
Without enough food to eat in the North Pacific, the birds would be unable to recover enough energy to successfully fly back to the Southern Hemisphere and breed.
"And on top of that, when the birds go to these places in the North Pacific, there's a potential for interaction with commercial fishing operationsgetting entangled in drift nets. It's almost a double whammy," he added.
Several studies suggest that sooty shearwater populations have declined in recent years, both at their New Zealand breeding grounds and at their feeding grounds off the California coast, Shaffer and colleagues report.
"These trends were associated with increases in oceanic temperatures, which may have limited regional biological productivity [of plankton and other one-celled organisms]," they write in their study.
According to the tracking data, the sooty shearwaters flew from their breeding grounds to just one of three wintering grounds in the North Pacific.
(Learn about sooty shearwaters in the U.S. in a bird-watching guide to the San Francisco Bay area).
They did not make a large sweep across the North Pacific, as an earlier study had suggested, Shaffer says.
"It makes far more sense," he said. "When they migrate to the North Pacific they molt, and when they molt, they can't fly as well."
Molting is the process of replacing old feathers with new ones.
During the migration, the birds fly fastest over the equatorial region, at times traveling nearly 620 miles (1,000 kilometers) in a day.
"It's pretty clear they're just trying to pass through the region on their way to better feeding grounds in the north," Shaffer said.
The timing and route of the northward trip varied among the birds, with different sootys crossing the Equator at different locations over the course of about a month.
The return trip, however, was remarkably synchronized.
All the birds funneled through a narrow corridor and crossed the Equator within a ten-day period in early October.
"That has implications for conservation down the road," Shaffer said.
For example, he says, the migratory flyway may require protection during certain times of the year to eliminate potential hazards to the shearwaters as they return to New Zealand.
Shaffer and his colleagues redeployed the tags this year, some on the same birds. The researchers plan to recover the tags in October.
A comparison of the data from year to year will help the researchers answer many new questions, like whether the birds travel to different feeding areas depending on food availability or if they always return to the same place.
Wikelski, the Princeton University professor, said such studies will allow scientists to understand how migratory animals think.
For example, the sooty shearwater study connects the birds' migration patterns with food resources.
"That means we get some glimpse into the animal mindhow it decides and what kind of knowledge it uses for these decisions," he said.
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