Swans have died or fallen ill during the past two years on Brayford pool in Lincoln,England.
One reason is that their feather were contaminated by oil leak in December 2003/
Second, they were infected with parasites called nematode worms.
But, experts were not sure about those reasons. Several more birds have died since, all of uncertain causes.
I am not an expert, but I think both of reasons are causes by which the swans died.
In Japan, many fish were killed by a crimson tide, which has two causes.
First is that the temperature of seawater becomes too high and the other is caused by emissious and waste.
So I think that whenever a problem happens, it has definite.
I think those problem are terrible. This causes is almost human activity. We should more careful about nature and animal, because human had destroyed those by now, so we should take responsibility.
I hope swans will not be more died.