The Friends of Magpie Bottom say that bright pink poison pellets have been put down by the badger sett at the nature reserve in Hanham. Two dogs ate some and had to be taken to the vets.
The incident has been reported to the local police beat officers.
Last spring there was a spate of anti-social behaviour at the reserve including youths armed with sling shots taking aim at ducks.
One duckling was found floating dead. Someone also mixed poison with bird seed and put it on the edge of the pond, raising concerns about harm to dogs, wildlife and children.