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Migration

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Wings and migration
One of the greatest benefits of flying for animals is that they can get access to distant sources of food that would be inaccessible to flightless animals. This is highlighted by the story of bird migration. Many kinds of birds travel thousands of kilometres to be in the right place at the right time for feeding on the bounty of that season.

Sandy Bartle, Curator of Birds, writes more here about the extraordinary achievements of long-distance fliers.

Arctic summer plenty
Birds that can fly well can make the most of widely scattered food sources. Migratory birds generally nest in safe places where the climate is not too harsh and where there is plenty of food to feed their growing chicks.

Bar-tailed Godwit on nest with chicks, Lapland

Bar-tailed godwit on nest with chicks, Lapland. Photo courtesy I E Hills
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If you look at the Earth’s globe, you’ll notice that most of the land is in the Northern Hemisphere, much of it in the continents that surround the Arctic Ocean. Though this ocean is mostly frozen, in summer the lands around it are not, and together they form the richest feeding grounds for birds anywhere. During the brief summer there is 24-hour sunshine and a super-abundance of food. But when winter comes, all the marshes, lakes, and seas freeze over, and for most birds no food is available for another six months.

Look at the examples of the bar-tailed godwit and the sooter shearwater/titi

Possession of wings allows birds to exploit this tremendous food resource. And when autumn comes to the northern lands, an estimated 15 billion birds fly south. Some just fly south to warmer places, like England and France; others migrate from Siberia to tropical countries like India, but many reach Australia and New Zealand, in search of an endless summer. These movements are not random. All Arctic migrants have favourite ‘winter quarters’, and many individuals return each year to the same harbour or bay in New Zealand.

 

Northern breeders flying south - kuaka

Migrant shorebirds, Germany. Photo courtesy Jan van de Kam
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Of the 45 shorebird species that migrate from the Arctic to New Zealand, the most famous is the bar-tailed godwit, known to Māori as küaka. Over 100,000 küaka migrate from their breeding places along the Arctic coasts of Alaska and eastern Siberia each year.

Recent research on the timing and weight of küaka leaving Alaska suggests that the southward flight of 11,000 kilometres from Alaska to New Zealand is non-stop, and that the birds fly high (4000-6000 metres) to keep cool. Under ideal conditions, the entire trip may take godwits just over 100 hours, compared with 14 hours needed for the same flight by jet airliner!

A tremendous accumulation of fat is the key. A record fifty-five per cent of godwit body weight is fat at the start, and the gut, liver, and kidney shrink.

References:
Barter, M. 2002. Shorebirds of the Yellow Sea. Wetlands International Global Series No. 9, Canberra.
Piersma, T. and Gill, R E. 1998. Guts don’t fly: small digestive organs in obese bar-tailed godwits. Auk 115: 196-203.

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