Anyone found killing a swan can faces a maximum £5,000 fine or imprisonment
Investigations are under way in Lincoln after several gruesome discoveries of swan body parts along a canal.
RSPCA officers and Lincolnshire Police said they had seen a rise in reports in the last year of swans being killed, possibly for food.
Since December, five birds have been found dead on the banks of the Foss Dyke canal.
Heads and feet have been found with the body missing and there was a report of a swan in a bin bag on Fouth Common.
Officers have been unable to find the people responsible.
PC Nick Willey, wildlife crime officer for Lincolnshire Police, said: "Since December we have had various reports of parts of swans being found on the river bank and on the cycleway, and it is quite clear that something is not quite right."
Swans are protected birds and the Crown retains the right to ownership of all unmarked mute swans in open water.
They are seen as a delicacy in some parts of the world but it is illegal to kill swans in this country and anyone found to be doing so faces a fine of up to £5,000 or a six-month jail sentence.
Charlotte Childs, branch manager at RSPCA Lincoln, said officers had been unable to gather enough evidence to bring about prosecutions.
She said: "It is extremely unpleasant finding the birds and it is impossible to know how many have been killed so so far, but it is a real big concern for us.
"The longer these incidents continue these people will keep getting away with it and the more damage they are causing to swans in the area."
The RSPCA and police have urged residents who notice suspicious activity along the canal to report it immediately.