The meltdown of the Fukushima Daiichi nuclear power plant may be responsible for a decline in New Zealand's muttonbird population.
A Department of Conservation study found only two-thirds of birds returned to an area near Auckland, after spending the northern summer in Japan - some only 20km from the plant, which was crippled in Japan's earthquake and tsunami in March last year.
The birds return to New Zealand in November to mate, but DOC seabird researcher Graeme Taylor told Radio New Zealand the ones that returned were in poor condition.
"We won't know if they've died up there in the north Pacific until another year goes by, because sometimes these birds skip a breeding season- where if they are in a poor condition they don't attempt to breed, and so they may turn up again and breed.
"But if the birds never turn up again then you have to start to wonder what's gone on with the population."
Mr Taylor said the research only looked at a small sub-sample of the breeding population, but it was the drop in numbers was the "most unusual event" in 20 years of studies of the birds' numbers.
He said many of the birds which arrived back had old feathers on their tails, wings and body.
"I've never seen birds in that poor of a condition come back to New Zealand."
He said the condition of muttonbirds suggested they did not get the food in the north Pacific they usually do.
The Fukushima disaster may be responsible, he said, although the La Nino weather pattern which lay over New Zealand last year may have been a factor.
Meanwhile a second study, undertaken Ngai Tahu and Te Papa, found 30 muttonbird chicks from the Ti Ti Islands near Stewart Island had no radioactive traces and were safe to eat.