Wellington  - Bar-tailed godwits - the long distance champions of the bird world - have started leaving New Zealand on their annual 18,000-kilometre migration to breeding grounds in Alaska.

The godwits (Limosa lapponica) spend the Southern Hemisphere summer at a number of estuaries and mudflats resting and gorging themselves to the point of being clinically obese in preparation for the flight to the other side of the world.

It is a remarkable feat of endurance, unmatched anywhere in the world.

Two years ago, New Zealand scientists monitored the flight of some birds using surgically implanted satellite transmitters to establish their flight path and habits.

They found that they flew 10,200 kilometres non-stop from New Zealand to the Yalu Jiang Nature reserve in China's Yellow Sea where they spent five weeks refuelling before flying another 7,300 kilometres to mudflats on the Yukon Delta in Alaska.

Fattened up after five months or so of little activity through the New Zealand spring and summer, they did not stop to eat or drink on the first leg of their journey.

But their numbers appear to be falling and there is concern that urban development in the Yellow Sea region of China and the Korean peninsula is encroaching on their stopover habitat and reducing the supply of food they need to continue to the breeding grounds in Alaska.

That concern is particularly felt in Christchurch, the largest city of New Zealand's South Island, where the birds are so revered that the bells of the cathedral are rung every September to announce the arrival of the first birds on the estuary where the Avon and Heathcote rivers meet, signalling the start of spring.

"The godwits are close to the hearts of Christchurch residents," said city council ranger Andrew Crossland, who organized a ceremony on Wednesday to show the city's affection for the birds as they began their long journey back to their Northern Hemisphere breeding grounds.

More than 500 people turned out to farewell the birds whose number spending the summer in the region has declined from a peak of 2,350 animals two decades ago. The population slumped to 1,390 in 2001 and although it recovered to 2,032 six years later, was down to 1,934 this summer.

Crossland believes many are perishing on their way home as the mudflats of their traditional feeding grounds in Asia are reclaimed for urban development.

"Loss of feeding habitat means that building up fat reserves for the final leg of the migration is becoming increasingly more difficult," he said.

"This makes our treatment of godwits in New Zealand even more important, as it is crucial that they set out on their return migration in absolutely peak condition."

Crossland said dog owners, particularly, had a responsibility to ensure that the birds were disturbed as little as possible on their summer stays in Christchurch.

"We need to ensure that godwits always have safe and secure feeding and roosting habitat on our New Zealand estuaries." (dpa)