Council officers are investigating an incident in which a swan was left decapitated.
The body of a male swan was found by dog walkers on the banks of a lake at Northlands Park in Basildon, Essex, on Saturday morning.
The head and neck of the swan were missing and bite marks were found by the base of the neck.
Sarah Moran, Basildon Council's animal control officer, said: "We examined the body of the swan on Saturday morning and it appears the swan was either killed or at least moved by a big dog.
"The bite marks were too big for a fox and the chances of a fox attacking a swan are very slim.
"What makes it worse is that swans pair for life and whilst I was at the park on Saturday, the female swan was looking for her mate and visiting the site of the attack."
Youths with their dogs were spotted in the park on Friday evening. Council and police officers are now trying to establish what happened.