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Migration

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Huge food resources are needed to accumulate such fat. Look at the globe again, and you’ll notice that most of the Southern Hemisphere is sea. No surprise, then, that most migrants which actually breed in New Zealand are seabirds. Fourteen species of seabird breed here and migrate to the North Pacific Ocean in winter to catch another summer. Of these, the most famous is the sooty shearwater, Puffinus griseus, known to Māori as tītī.

About 10 million tītī breed on islands around and south of New Zealand in summer. Males and females take turns at incubating the egg, each shift being normally 4-16 days. As the incubating bird fasts its partner is away at sea feeding. During this time some adults make their way up to 1700km south to feed on the shrimp-like krill of Antarctica’s Ross Sea. Krill are best known as whale food.

In May the entire sooty shearwater population migrates 12,000 kilometres to the North Pacific Ocean. Some birds even reach Arctic waters, but most occupy cold waters from Japan across to California. Before their return flight in September tītī are able to feed on oil-rich anchovies, which at that time reach peak abundance off California. Their stored fat increases to over 40 per cent of their weight then, enough for the long non-stop journey back to their favoured nesting site in New Zealand.



Try to imagine the energy implications of migration. The energy consumed by the 10 million returning shearwaters is equivalent to 10 million 500-gram margarine containers of fat! Without super-abundant food sources, neither the godwit nor the sooty shearwater could make such journeys. Although migrants are able to exploit food resources on a huge scale, they are very dependent on the food being sufficiently abundant in the right place and at the right time. Global warming, habitat change, and competition with fisheries are all potential long-term threats to these spectacular journeys, some of the longest made by any species.

Finding out about migration

Jean-Claude Stahl from Te Papa checks a Buller’s albatross on Snares Is. Photo courtesy Kennedy Warne

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How is migration studied? The basic method is to mark as many birds as possible with individually numbered metal bands. Anyone finding a dead migrant can then return the band to the country of origin. Over many decades a good picture of the routes and timing of godwit and sooty shearwater migration has been built up through the return of bands. Of course, returning bands is entirely voluntary - there are no rewards - so most of our basic knowledge of the migrations of birds has been accumulated by the cooperation of ordinary people all over the world. Anyone who returns a band contributes vital information on the life of the bird.

All New Zealand bands carry the name and address of New Zealand’s national museum, Te Papa, where the Department of Conservation’s Banding Scheme was originally developed.

In recent years Te Papa researchers have been studying the migrations of seabirds by fitting the birds with satellite transmitters. These send out pulses that are picked up by passing satellites, which then fix the bird’s position. Unfortunately the lightest satellite transmitters still weigh more than 30g with the battery, which is too much weight for a smallish bird like a godwit or shearwater to carry for a long period. So Te Papa researchers have concentrated on tracking small albatrosses like this Buller’s albatross, shown with Jean-Claude Stahl at its nest on the Snares Islands, south of New Zealand.

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