The success of a wild bird in Scotland despite declining numbers in the rest of Europe has mystified experts.
A red-throated diver pictured on Shetland
RSPB Scotland said it was delighted but puzzled by breeding figures for the red-throated diver.
The rarer black-throated diver is also on the increase, possibly thanks to the anchoring of man-made rafts in lochs.
A survey of divers by the RSPB and Scottish Natural Heritage (SNH) found the two species had increased in the UK by 16 and 34% in the last 12 years.
Both have declined in Europe and the black-throated diver was last week made a conservation priority by the UK government because of the declines elsewhere.
In Scotland, its numbers rose from 187 pairs in 1994 to 217 pairs in 2006.
In the Highlands - their stronghold - they were declining because some nests were being drowned in floods while eggs at other sites were lost to collectors and predators.
The new study shows the greatest increase in the Western Isles, but also improved figures in the Highlands.
A total of 58 rafts have been installed on remote lochs in the region. They protect the birds from flooding and animals that prey on them and their eggs.
Stuart Benn, senior conservation officer for the RSPB, said: "We can't say hand on heart that the overall increase is due to the rafts because we haven't ringed the chicks, but there is no doubt that the rafts have turned out to be very, very good at what they do."
The RSPB said it was a mystery as to why red-throated divers had done so well.
Its numbers have risen from 935 to 1,255 breeding pairs in 12 years.
However, in Shetland the population has dropped from 700 pairs to 407.
The red-throated diver is steeped in mythology and is known as the rain goose in Orkney and Shetland.
In the 19th Century, it was regarded as a foreteller of storms in many parts of the world.
Dr Mark Eaton, an RSPB scientist, said: "We feared the numbers of red-throated divers might drop because the warming of the North Sea seems to be reducing stocks of the fish they feed on.
"The black-throated diver could also be at risk in the future, despite the recent increases. If climate change causes loch temperatures to rise, the small fish the birds feed on could grow too large to eat."