The RSPCA has asked the public to be vigilant after more than 20 swans died on a stretch of river in County Durham in the past few weeks.
It was unclear whether the birds that live on the River Wear in Chester-le-Street were being deliberately or accidentally poisoned, or if they have succumbed to natural illness.
There are about 100 swans on that section of the Wear, which passes through Riverside Park, and it is popular for children to feed the inquisitive swans. WORDS: PA
A sign that was erected some time before the recent spate of deaths asked the public not to feed the birds mouldy bread as it is poisonous to swans.
But it was not clear whether that was the cause, and tests will be carried out.
RSPCA Chief Inspector Michelle Charlton said: "Just over 20 swans have been found dead, or so sick that they've had to be put to sleep, on the river in Chester-le-Street over the past few weeks.
"Calls started coming in about them in January and RSPCA officers have picked several up over the past few days.
"Tests are being conducted on the birds but at the moment we have no idea what the cause is, whether it's the result of a deliberate act or a naturally occurring illness.
"We'd like to appeal to the public to be vigilant, and if you come across anything that you think might be being dumped into the water to contact us."
Video: Bloodied swan rescued from field
Black swans feed carp in amazing picture
Swan attacks tourists and rowers on River Cam