Scientists are scrambling to understand the sudden death of an estimated 20,000 guillemots off the Dutch coast, hundreds of which are washing up on the country’s shoreline.

The bodies of the birds, which spend most of their lives at sea where they dive for their food, started emerging over the past month, from the Wadden Islands in the north to Zeeland in the south.

Mardik Leopold, a biologist from Wageningen University, said the Netherlands “had not seen such mass deaths since the 1980s and 1990s”, describing it as “rather a major incident”. Hundreds more sickly birds have been taken into sanctuaries for treatment.

“The working…