Two swans were shot in the neck at a Reservoir in Claydon.
They were found beside Clattercote Reservoir and reported to the police and RSPCA on September 14.
The charity only released details of the killings on Wednesday.
The RSPCA’s inspector Annabelle Black said: “We really want to find the person responsible for the swans’ deaths and I would appeal to anyone who may have seen anyone acting suspiciously in the area, or who has heard of people taking pot shots at swans.”
She urged anyone with information to call the charity on 0300 1234 999.
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