We have had problems with losing adult swans at one particular site, a large shallow lake, but have not been able to come up with a cause. Many of the swan deaths are from fox attacks, but only after the swans have become very thin and out of condition. Other swans displaying the same symptoms have been rescued, some of which responded to treatment which includes being wormed, others do not and die. At the moment, the mortality rate is almost 50%.

The main symptom is significant weight loss, but the swans still retain or have an increased appetite. Indeed, they seem to forage even more than usual and move on to the fields surrounding the lake. This is when they become vulnerable to fox attacks, especially as they are in very poor condition by this time. It seems like the more they eat, the thinner they become.

The latest problems occurred at the end of the summer just after the swans had completed their moult, but we have had more fatalities in October and November. There are Canada geese on the same lake which do not display any of the above problems and the water has been tested for pollutants and found to be clear.

I would greatly welcome your opinion.