Bird-brained behaviour: The ultimate migration

When scientists fitted a tracking device to a small brown seabird, they knew it thought nothing of crossing the Pacific. But their research has proved that the sooty shearwater is capable of an astonishing 39,000-mile odyssey in search of an endless summer. Steve Connor reports

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For such a small seabird, the sooty shearwater has an ambitious take on the world. Despite its diminutive size, it thinks nothing of flying from New Zealand to Alaska in pursuit of an endless summer.

For years, ornithologists have known that sooty shearwaters breed off the coasts of New Zealand and Chile in the southern hemisphere, and then cross the equator to the rich summer feeding grounds of the North Pacific, which stretch from California to Japan. Now a study has shown that this epic feat is undertaken over a single breeding season, with individual birds travelling as far as 39,000 miles in just one year.

It is the longest migration route undertaken by individual animals that has been recorded by scientists, according to Scott Shaffer, a research biologist at the University of California, Santa Cruz, who led the team behind the study. "The only 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," Dr Shaffer said. "But we don't know if they do that in a single season, because nobody's ever tracked them."

The latest research into the migratory routes of the sooty shearwater has given a clear picture of the birds' extraordinary journeys, including distances, speeds, stop-over points and even the variations in air and sea temperatures they encountered.

The miniaturisation of sophisticated electronic tags has enabled the scientists to place them on smaller and smaller birds without interfering with their ability to fly - or indeed swim, in the case of other migratory creatures such as sea mammals, fish and turtles.

Each tag weighs less than half an ounce, and the scientists managed to recover the devices from 20 birds at two breeding colonies in New Zealand.

The tags recorded daily activities during the migration and return journey. As well as recording geographic location, they measured temperature and pressure, which gave the scientists an unprecedented insight into how and when birds would dive for their food.

It is all part of the wider study into some of the immense odysseys undertaken by many species of animals in the search for food and suitable breeding sites - or just somewhere to escape the long winter nights.

Shearwaters feed on fish, squid and shrimp-like krill, all of which can dramatically increase or decrease in abundance depending on the time of year, with winter in each hemisphere normally being a time of scarcity.

Dr Shaffer and his colleagues found that the migratory pattern of the sooty shearwater was optimally geared towards exploiting the seasonal availability of food in the North and South Pacific.

These small seabirds cross the equator twice a year in a figure-of-eight pattern to chase the summer, when their feeding grounds are nearly always at or near the period of peak productivity, Dr Shaffer said. "When they cross the equator, they're travelling fast and not stopping much to feed. They feed near Antarctica during the austral summer, then they zip north to feed in one of three areas of the North Pacific, taking advantage of high [feeding] productivity throughout the year," he explained. The electronic tags used in the study, which is published in the research journal Proceedings of the National Academy of Sciences, recorded shearwaters diving to depths as great as 68 metres (225 feet) in pursuit of their underwater prey.

One of the most fascinating findings to emerge from the research was that sooty shearwaters do not roam far once they have reached their final destinations in the northern or southern hemispheres. Before this study, it was assumed by many experts that shearwaters roam widely in between their trans-equatorial passage. Yet the scientists found that each bird heads for just one destination in the northern hemisphere, and stays there until it heads south again.

Intriguingly, there is no apparent reason why some birds in a colony travel to one destination in the opposite hemisphere, while others in the same group head for another. A colony breeding off New Zealand or Chile can split up after raising their young, with individual members spending the next six months in any one of the three shearwater feeding grounds in the north - from Japan to Alaska.

Even members of the same breeding pair, or siblings raised in the same nest, could end up spending their summer months thousands of miles apart, Dr Shaffer said.

Another discovery to emerge from the study was that the timing and the route of the northward migration was variable, with birds crossing the equator at various locations over a period of about a month. But the return trip south was remarkably synchronous. All of the tagged birds funnelled through a narrow corridor and crossed the equator within a 10-day period in October, Dr Shaffer said.

Shearwaters also take advantage of prevailing patterns of wind circulation and the Coriolis effect of the Earth's rotation - which influences the direction of the winds immediately north and south of the equator.

At the start of their migration in early April, the shearwaters, which are in the southern Pacific at this time of the year, travelled eastwards in the direction of prevailing westerly winds.

Once they began to head north, they appeared to use easterly trade winds to fly north-west over the Pacific at maximum migration speeds, which enabled the birds to cover more than 100 miles in a day. Once they crossed the equator and found themselves in the cooler waters of the North Pacific, the shearwaters headed apparently at random for one of three possible destinations - Japan and the north-west Pacific, Alaska and the north-east Pacific, or California.

"The figure-eight pattern is completed when birds return to New Zealand waters, again purportedly by using easterly trade winds in a southwesterly direction," the researchers wrote in their report. "Remarkably, travel of all birds across the equator is highly synchronised and passage is through a narrow corridor," they wrote.

And all this to avoid winter. As the researchers conclude: "The tracking data reveal that sooty shearwaters experience a perpetual cycle of spring, summer and autumn from year to year."

Five other marathon migrants

Albacore Tuna

These fast-moving fish travel across the North Pacific, with spawning occurring somewhere off the south of Japan. They are also found from Canada to Mexico. All are believed to derive from the same breeding stock.

Monarch Butterfly

Perhaps the most impressive migrant of all. It overwinters in Mexico and flies north as far as Canada to breed. Successive generations then fly south, often returning to the same tree as their parents or grandparents.

Fin Whale

Migrates from feeding grounds in the polar seas to calve in the subtropical oceans. It can travel at speeds of up to 23mph, which protected it from slower whaling vessels prior to the advent of factory fishing.

Black-footed Albatross

Like other albatrosses, this species is an impressive flyer that can remain at sea for years at a time without ever touching land. It migrates between California and Alaska and breeding grounds in northern Hawaii.

Leatherback Sea Turtles

The most endangered sea turtle. Studies of its migratory routes could help conservation efforts. Has the widest distribution of any reptile and crosses the equator regularly to feed in both northern and southern hemispheres.

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