Dutch scientists baffled over 20,000 seabirds dead or dying washing up on North Sea

"What's killing them though is the million-dollar question," Mardik Leopold, a maritime researcher for Wageningen University, told AFP.

"And we still don't know what the answer is. It's an alarming situation," he added.

"The last time we saw high mortality rates like this was in the 1980s and 1990s."

A number of questions are puzzling scientists, he said.

The bird deaths are confined only to Dutch shores -- nothing has been reported in Belgium or Germany.

High winds and stormy winter seas also could affect the birds' feeding patterns on herring and sprat as they become too fatigued to eat, Leopold said.

"But again, why are the deaths only localised to the Netherlands? Surely we're not the only place experiencing winter weather?" said Leopold.

Dutch media have raised the question of whether the deaths may be linked to a recent container spill, littering the Dutch and German coast lines with debris, including plastic toys, polystyrene, shoes and at least one bag with a dangerous powder identified by authorities as "organic peroxide."

But Leopold said an initial autopsy on a small number of birds showed "no plastic" in their stomachs.

Similarly, if the birds were affected by a chemical, other animals would also have shown symptoms, the marine biologist said.

"The birds were not covered in oil either," he added.

Dutch authorities said Wednesday they had been told by the MSC shipping company that at least 341 containers were lost off the MSC Zoe as it was battling a storm last month.

Scientists are now planning a mass autopsy of hundreds of dead birds next week, hoping to shed more light on the mystery.

However, the species is not threatened with extinction. Some two million guillemots live in the North Sea, the NOS public broadcaster said.