By Laura Sheeter
BBC News, Estonia
International volunteers and Estonian conservationists are trying to save the last surviving birds caught in an oil spill in the frozen Baltic Sea.
The authorities still do not know who caused the oil spill
The slick appeared off Estonia's north-west coast in late January.
"I'm wearing four layers of clothes, a down suit and a dry suit, and you can still feel the cold through all that," says one rescuer standing on the ice.
"But what else can we do? We can't just leave the birds to die."
The spill, the source of which is still unknown, is estimated to have killed 35,000 birds, making it by far the worst Estonia has experienced.
Estonian prosecutors say it is mostly fuel oil, deliberately washed out by a tanker.
However, despite travelling to Egypt to take samples from one suspect tanker, investigators say it may be impossible to ever find those responsible.
Although not very big in global terms, the spill's location - close to important wintering sites for sea ducks and swans - and the extremely cold winter weather mean there have been few survivors.
Slick made up of estimated 20 tonnes of fuel oil
3,500 dead birds collected, but 35,000 birds thought to have died
Swans and sea ducks most seriously affected
Oil-covered feathers lose their waterproofing, and with temperatures down to -20C, birds rapidly die of hypothermia.
Weeks after the spill, the only oiled birds still alive are swans. The rescuers have to wait for the birds to become weak enough to be caught, by which time their chance of recovery is much slimmer.
Mark Thompson, from the British Royal Society for the Prevention of Cruelty to Animals (RSPCA), is leading the rescue.
"We've caught maybe 500 birds, but in these temperatures it's really hard. More than once we've had to cut birds out of the ice when they've become frozen into the sea," he says.
But news that the deadly H5N1 strain of bird flu had been found in the Baltic Sea off Germany brought an abrupt halt to the search.
The Estonian authorities decided that thse risk was too great for those trying to rescue and treat the birds, so no more survivors would be caught.
The work continues, though, to treat those who have been rescued.
An hour's drive inland at the temporary treatment centre in Keila, more than 100 swans, long-tailed ducks and golden-eyed ducks are being warmed up, fed and washed.
Many arrive totally emaciated, and have to be fed until they regain their winter weight and are strong enough to be released back into the wild.
The full extent of the damage is still not known
The vets at the centre have come from all over the world and specialise in treating oiled birds.
Dr Ian Robinson, from the International Fund for Animal Welfare, is leading the team and fears that although the rescue operation is now over, there will be more victims of this oil spill.
"We have something of a time bomb here," he says.
"At the moment, the oil is trapped under the ice, so you can't see it, and you can't clean it up. When the sea thaws, that oil will be released, just at the time when all the migrating birds are returning here from the south.
"I have no doubt we'll see more birds caught in this spill."
But come the spring, and things could be even harder, as the team may not be allowed to rescue any birds at all, if the risk from bird flu is still thought to be too great.