The swans have suffered from a mysterious illness

Tests have failed to show why so many swans on Brayford Pool in Lincoln have died or fallen ill during the past two years.

Nine swans died in the space of a week in early 2004 and some of the deaths were linked to a suspected oil leak.

Lincoln City Council, the RSCPA and the Environment Agency will meet in a few weeks to decide what action to take.

Nematode worms

Chief Inspector Mike Hogg from the RSPCA said: "We are concerned as swans in other areas of the region are bigger and healthier than those at the Brayford."

Dozens of birds were taken out of the water shortly after an oil leak in December 2003 because their feathers were contaminated.

It meant they were no longer waterproof and they were in danger of catching hypothermia.

Several died but experts were not sure if the cause was pollution or a parasite called nematode worms.

Several more birds have died since of uncertain causes.