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A record-breaking godwit known as E7 is refuelling in the Firth of Thames after having made it all the way to Alaska and back wearing a surgically implanted satellite transmitter.

The female bird was the first of 16 bar-tailed godwits tagged in February by ecologist Dr Phil Battley, from Massey University, to return to New Zealand.

Data provided by the transmitter meant that Dr Battley could confirm her route, with her entire migratory journey clocking in at close to 30,000 km, and the southern return leg at more than 11,500km.

Dr Battley said E7 set the record for the longest flight of the tagged birds on the journey north and then broke her own record on the way back south.

E7 first went to the Yellow Sea, flying 10200 km direct from Miranda in the Firth of Thames, where she was banded up, to Yalu Jiang Nature reserve in China.

She spent five weeks refuelling there and then at the start of May flew to the breeding grounds in Alaska, another 7300 km.

Dr Battley said E7 went to the Yukon-Kuskokwim Delta and spent the next two months there where she was almost certainly breeding.

"She left there about mid-July and went to the mudflats on the edge of the Yukon Delta where she refuelled again, getting nice and fat until the end of August."

Dr Battley said the southward migration of the godwit from Alaska to New Zealand was thought to be the longest non-stop migration of any bird.

"She had the option to fly down to the Alaskan peninsula and take off from about 500 km further south but she didn't do that - this indicates the long journey is not such a problem to her or that she's needing to find a shorter route."

Dr Battley said E7 was back at her favourite spot at Miranda after becoming the first godwit to have her migration monitored by satellite, but had confounded attempts to photograph her after her epic journey.

"Unfortunately it's a muddy spot with no access so while it would be nice to have pictures we just haven't been able to photograph her."

Dr Battley said E7 probably arrived late on Friday night.

Her transmitter was 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 E7 was recorded as being back at Miranda.

She was expected to rest in the Firth of Thames until about March, when she would make her way back to Alaska to have her chicks.

E7 would have a complete feather moult before starting out all over again. "She'll moult into breeding plumage, fuel up again, and take the same route back to the Yellow Sea and Alaska.

"What's really amazing is that once her chicks have fledged they'll be left to their own devices and will have to migrate to New Zealand without parental guidance."

Dr Battley was now awaiting the arrival of four other birds with transmitters that were still working. Eight birds fitted with backpack tracking devices had not been monitored because the devices appear to have fallen off.

The transmitters on three of the eight birds which had the devices surgically implanted also appeared to have stopped working.

Godwits arrived in New Zealand in September each year and the adults left in mid-March, with adolescent birds staying until they were up to three or four years old.

The largest populations in New Zealand were found in the Kaipara Harbour, Manukau Harbour and Farewell Spit.

Dr Battley's next project was to undertake similar work with a sub-population of the bar-tailed godwit in northwest Australia, allowing comparison of the migratory habits of the two populations.

The project would provide crucial information about the migratory behaviour of declining species.

The satellite track of the godwits' travels can be viewed at: