They also revealed that large amounts of oil were also menacing the other nearby island sanctuary of Skokholm.
Around 50,000 birds - mostly guillemots and razorbills - are at present on the two islands, but tens of thousands more from all over the world will arrive in the next few weeks.
"The lives of 500,000 migratory birds are now seriously at risk," said Judy Phillips, of Dyfed Wildlife Trust, which runs the nature reserves on the islands. "We are dismayed and angry that Sea Empress has been almost complacently allowed to continue spilling its cargo for almost a week.
"The Government has failed to provide an adequate and rapid response to what is turning into an ecological disaster before their eyes."
Thick black oil lapped ashore on Skomer during Tuesday night from a slick estimated to be 250 metres long. Simon Smith, resident warden on the island, said he feared high casualty numbers because many unsuspecting sea birds were now coming ashore because sea and weather conditions had improved after recent storms.
Simon Lister, the Dyfed trust's director-general, speaking in Milford Haven, said: "This whole affair has been little short of a national disgrace. Nothing like the efforts needed have been put into dealing with this properly. If you ask who is to pay for all this, the answer is that wildlife is paying."
A squadron of seven aircraft have continued to spray three main oil areas along the South Pembrokeshire coast- one of them now 25 kilometres long.
Tons more chemical dispersants arrived by truck at an airfield near Haverfordwest as part of the international multi-million pound salvage and clean-up operation. A C130 aircraft, the US equivalent of the Hercules, yesterday joined the spraying operation.
Meanwhile, Kevin Colcomb, a government pollution scientist, said spraying was proving effective because of favourable weather. But only about 1,000 tonnes of the spilt oil has so far been recovered from beaches along the Milford Haven estuary.Reuse content