A white duck was decapitated last year near the park where a black swan was found with its head removed on Thursday.
Stirling resident Margaret Goode said yesterday she was horrified to find the duck at the Princeton Private Estate last June and this week's attack was eerily similar.
"Its head was completely gone," Ms Goode said of the duck. "There was no damage to the body so I don't think it could have been caused by another animal."
An RSPCA spokeswoman said the success of its investigation into the swan's death would hinge on information from the public. "We are hopeful that given the bird was well known to the community someone is going to know what happened," the spokeswoman said.
Anyone with information is asked to call 9209 9300.