Monday, September 10, 2007

Bar-tailed Godwits on the Move

One of the posts submitted to last week's I and the Bird discussed a project to map the migration tracks of three shorebird species: long-billed curlew, bristled-thighed curlew, and bar-tailed godwit. The mapping relied on fitting a handful of each species with satellite transmitters, which broadcast each bird's location every 36 hours. At the time Kevin wrote that post, the two curlews had left, but the godwits were still in Alaska.

As of yesterday, the first of the bar-tailed godwits, designated E7, had reached her winter home in New Zealand after competing the longest recorded nonstop migration flight.

E7 is the first godwit to have her full annual migration monitored by satellite. It included a southern return leg of more than 11,500km -- the longest non-stop flight by a bird to be recorded.

"From the speed that she was going, I'm absolutely confident that she came direct," said Massey ecologist Dr Phil Battley, who tagged 16 bar-tailed godwits to identify how they made their way to and from Alaska.

The south side of the Firth of Thames, near Miranda, was a muddy spot with difficult access, so it had not been possible to photograph the bird, which arrived l ate on Friday night.

Her transmitter switched itself on for six hours every 36 hours and on Friday afternoon she was south-west of Ninety Mile Beach in Northland. By 3am on Sunday morning she was back at Miranda where she is expected to stay " resting and refuelling" until about March, when she will make her way back to Alaska to lay eggs.

Dr Battley said E7 took off from the Yukon delta and could have shortened her journey by moving down to the Alaskan Peninsula to take off from about 500km further south.

"But she didn't do that," he said. "This indicates the long journey is not such a problem to her".

"It's quite amazing that even on a journey of 11,500km she's not trying to make it any shorter. She's got enough in reserve to cope.
You can view the updated migration tracks for bar-tailed godwits at the USGS Alaska Science Center. If you download the update, it should open in Google Earth. I have posted screen grab for today's update below (click to enlarge). E7's track is in red.

The route passes close to a few islands, such as the Hawaiian archipelago. But as you can see from the image, E7 flew that distance mostly over open ocean. Such a migration route leaves birds with very little margin for error. According to BirdLife, bar-tailed godwits often live more than 20 years. That they can survive that long while flying over 29,000 km annually is amazing.