Study documents marathon migrations of sooty shearwaters

Every summer, millions of sooty shearwaters arrive off the coast of California, their huge flocks astonishing visitors who may have trouble grasping that the dark swirling clouds over the water consist of seabirds. Scientists have long known that sooty shearwaters breed in New Zealand and Chile and migrate to feeding grounds in the Northern Hemisphere. But the details of this remarkable transequatorial migration are only now emerging from a study using electronic tracking tags to follow individual birds.

The flights of sooty shearwaters documented in this new study represent the longest animal migration routes ever recorded using electronic tracking technology: around 65,000 kilometers (39,000 miles). Taking advantage of prevailing winds along different parts of the migration route, the birds trace giant figure eights over the Pacific Basin.

“The only other bird species known that could rival the migrations of the sooty shearwater would be the arctic tern, which breeds in the Arctic and migrates to Antarctica. But we don't know if they do that in a single season, because nobody's ever tracked them,” said Scott Shaffer, a research biologist at the University of California, Santa Cruz, and first author of a paper describing the new findings, published in next week's online early edition of Proceedings of the National Academy of Sciences (PNAS).

Shaffer worked with an international team of scientists from UCSC and other institutions in the United States, New Zealand, and France. The researchers found that the migratory cycle of sooty shearwaters encompasses the entire Pacific Basin, focusing on the richest feeding grounds in both hemispheres–from Antarctic waters in the south during the breeding season to bountiful coastal currents off California, Alaska, and Japan in the north. These small seabirds cross the equator twice a year in pursuit of an endless summer in which their feeding areas are always at or near the period of peak productivity.

Diving patterns recorded by the tags indicate the birds stop little if at all to feed as they pass through the equatorial regions on their journey between the Southern and Northern Hemispheres.

“When they cross the equator, they're traveling fast and not stopping much to feed. They feed near Antarctica during the austral summer, then zip north to feed in one of three areas of the North Pacific, taking advantage of high productivity throughout the year,” Shaffer said.

Sooty shearwaters are one of the most abundant bird species in the world, with a total population estimated at about 20 million. Nevertheless, they are potentially vulnerable to changes in their food supply, Shaffer said. Scientists have reported recent population declines at breeding colonies in New Zealand and in the eastern North Pacific.

The shearwaters feed on fish, squid, and shrimplike krill, which they take from the surface or pursue underwater. The electronic tags recorded birds diving to depths as great as 68 meters (225 feet) to capture their prey. Average dive depth was about 14 meters (46 feet).

The new study shows that, contrary to previous assumptions, sooty shearwaters do not make a big pan-Pacific sweep to cover all of their feeding areas in the Northern Hemisphere. Instead, individual birds went to just one of the three major hot spots and stayed there until it was time to return south to breed. But Shaffer said the birds that travel to different regions do not represent distinct populations of shearwaters.

“Birds that came to California stayed in California, and if they went to Japan they stayed there and then returned to New Zealand. But two birds from the same nest can end up going to opposite sides of the Pacific, and birds from different breeding colonies can end up in the same place,” he said.

The timing and route of the northward migration was somewhat variable, with birds crossing the equator at various locations over a period of about a month. But the return trip was remarkably synchronous, Shaffer said. All of the tagged birds funneled through a narrow corridor and crossed the equator within a ten-day period in October. Ongoing research using tags redeployed on the same birds will show whether or not individual birds have preferred feeding areas that they travel to each year.

The research on sooty shearwaters is part of a Census of Marine Life project called Tagging Of Pacific Pelagics (TOPP), which is deploying electronic tags on 23 species of top predators in the North Pacific Ocean. Daniel Costa, a professor of ecology and evolutionary biology at UCSC who oversees TOPP studies of marine mammals and seabirds, said the shearwater study was only possible because of the increasing miniaturization of electronic tags.

“When we first got together to plan the TOPP program, we didn't know if the technology was available to tag a bird that small,” Costa said. “Sooty shearwaters were a species that we knew came here from the Southern Hemisphere, but we knew very little about its migratory patterns. Now we know this is an organism that integrates resources across the entire Pacific, which is quite a feat when you think about it.”

The tag used on the sooty shearwaters, made by a Canadian company called Lotek Wireless, weighs about 12 grams (less than half an ounce). The researchers recaptured 20 tagged birds at two breeding colonies in New Zealand. The tags recorded the daily activities of each bird during part of the breeding season and throughout the migration to the Northern Hemisphere and back. In addition to geographic locations, the tags recorded pressure and temperature data, providing information about the birds' diving behavior and the temperatures they encountered at sea.

The temperature data helped Shaffer's team ensure the accuracy of the location data, using a technique that integrates temperature readings from the animals with those obtained by orbiting satellites. This method was developed in the lab of TOPP principal investigator Barbara Block of Stanford University and validated by Shaffer's team in studies of albatrosses.

Collaborators on the project also included seabird expert and TOPP investigator Henri Weimerskirch of the French National Center for Scientific Research (CNRS). He persuaded researchers in New Zealand who have been studying the shearwater breeding colonies to join forces with the TOPP researchers.

“One of the nice things about TOPP is that we have a consortium of some of the top scientists in the world, which makes it easy to make those kinds of connections,” Costa said.

In addition to Shaffer, Costa, Block, and Weimerskirch, the coauthors on the PNAS paper include Yann Tremblay, a postdoctoral researcher at UCSC; Darren Scott and Henrik Moller of the University of Otago, New Zealand; David Thompson and Paul Sagar of the National Institute of Water and Atmospheric Research, New Zealand; Graeme Taylor of the Department of Conservation, New Zealand; and David Foley of the University of Hawaii.