Two birds were beheaded at a nature reserve in Greater Manchester, it has emerged.
A mute swan and a coot were caught on a fishing line at Pennington Flash Country Park in Leigh, before being decapitated.
The attack happened at about 1830 BST on 2 September.
Police released details to try to trace a man who told park rangers he had seen two men trying to catch wild birds but left without giving his details.
Pc Chris Clarke said: "These birds have suffered horrific deaths and all right-thinking people will no doubt be appalled.
"They are beautiful wild birds that are protected under law. Whoever carried out this needless slaughter clearly intended to cause as much suffering as possible."
He said the man who spoke to the park rangers had "vital evidence" which could help convict the attackers and urged him to contact police.
A spokesman for the RSPB said: "This is a cruel, callous and serious crime. If caught, the perpetrator could be given up to a £5,000 fine and/or six-months in prison."
Three men, aged 44, 40 and 23, were arrested on suspicion of killing a wild bird contrary to the Wildlife and Countryside Act 1981. They have since been released without charge.