Several seabirds have been rescued off the coast of North Holland province after apparently being caught up in an oil spill.

Six guillemots and a gannet have been found contaminated with oil off the coast of Texel in recent days, while two more were brought in to a sanctuary in Den Helder.

One bird is said to be 80% covered, while the others had traces of oil on their undersides. Sanctuary workers have to wash and rinse the birds repeatedly with warm water and soap until the oil is gone.

The oil damages the birds’ feathers and makes them less waterproof, leaving them vulnerable to sinking or drowning.

No leaks have been reported in the North Sea, but the coastguard is investigating the source. Beachgoers on the island have been warned to look out for affected birds and report any sightings to the Ecomare wildlife centre.

Jarco Havermans of Ecomare told local media site NH Nieuws that the incident came as a “complete surprise”.

“I know that there was one winter in the 1990s when we took in around 100 birds,” he said. “Now and then a bird comes in which has been smeared in something. Thankfully we don’t see oil-covered birds so much any more.”

Oil spills have been less common in recent years since chemical labelling made it easier to trace leaks and reclaim the clean-up costs from shipping companies.