As hundreds of emaciated corpses continue to be washed up on East Coast beaches from Yorkshire to Shetland each day, the society has raised its estimates of the total killed to about 75,000.
Those that struggle ashore still alive soon die. Although sustained easterly gales have played a part in the disaster, wildlife conservationists fear over-fishing could also be a cause.
Barbara Young, the RSPB's chief executive, said: 'It's much bigger than anything we've experienced before and we've got to try to find out what's going on. We're hearing various stories about birds moving in strange directions and feeding in strange places.'
There have been mass deaths before on the British and continental North Sea coastlines; the phenomenon could be natural.
The guillemot, an abundant species with up to two million in the North Sea, is by far the worst affected, with razorbills, shags, cormorants, kittiwakes, puffins and little auks also dying.
Prolonged rough seas may have stopped the birds diving for their food in the turbulent waters or have forced their prey deeper. Easterly winds may have driven starving birds and corpses on to the East Coast which would otherwise have remained at sea.
But up to one million tons of sand eels - a key food for the birds - have been taken from the North Sea each year in recent years.
The catch, which hardly existed before the 1970s, is the largest in this sea and Denmark and Norway are responsible for almost all of it. There are no international agreements to limit the fishing.
European fishery experts estimate half of the pencil-sized mature sand eels in the northern North Sea are taken by fishing rather than natural predators.
Danish boats have been taking them from the Dogger and Fisher Banks and from nearer to the Scottish coast.
They are unsuitable for human consumption and are processed into fishmeal and oil. At one time this oil was even being burnt in a Danish power station.
Danish and Norwegian boats also take about 100,000 tons of sprat each year - another small fish which is a key part of seabirds' diet.
Sand eel fishing by local boats around Shetland was blamed by conservationists for the collapse in the number of seabirds breeding on the islands in the 1980s.
Because the fish stocks also showed signs of collapsing due to over-fishing the fishermen brought in voluntary restraints, followed by compulsory ones introduced by the Scottish Office in 1989. Since then bird numbers have shown some signs of recovery. Shetland has also had the highest number of carcasses washed up in the past few weeks - some 50,000. This is more than 30 times the number known to have been killed by last year's Braer oil spill.
RSPB officials in Inverness yesterday described waters in the Beauly Firth 'surging' with 10,000 razorbills and shags which had flown 20 miles from their normal feeding grounds to find a shoal of herring sprats.
David Mitchell, RSPB spokesman, said: 'This seabird disaster - with birds being funnelled inland in a desperate effort to find food - is on a quite unprecedented scale.
'In 1983 30,000 birds died over a four or five week period of severe weather when they were blown inland. Now we have almost three times that number dead in just a fortnight. On some beaches there are corpses everywhere you turn.'
Adult birds which would normally be building up their fat stocks at the start of the breeding season have been badly hit, Mr Mitchell said. 'They are washing up, their breastbones sticking out and their digestive systems empty . . . the effect on the overall population will be felt for years.'
The Scottish Office said there was no evidence that over-fishing was responsible for the deaths; sand eel stocks appeared to be holding up despite the huge catches.
Chris Mead, of the British Trust for Ornithology, said: 'There's no reason as yet to suppose the primary cause is over-fishing.'
This weekend up to 1,000 volunteers will walk some 2,000 miles of British coastline in an annual count of dead seabirds. Although some starving birds will be found alive, most will be dispatched on the spot - in their weakened and shocked condition they rarely survive rescue.
In fact surveys indicate the numbers of guillemots doubled between 1970 and 1987 with over-fishing possibly boosting their population. Decline in herring, cod and haddock stocks may have allowed the sand eels these larger fish eat to increase in number, supporting more birds.
British fishermen are now putting pressure on the Government to curb the Danish sand eel fishing because they fear it could be removing much of the food the cod and haddock depend on.
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