A gang of teenagers were seen pelting sticks and rocks at a nesting swan, police said.
The group was said to have attacked the bird in the Abbey Grounds, Cirencester, on Sunday evening.
A Good Samaritan disturbed the teenagers and came to the aid of the swan, which was uninjured.
Officers have since launched an investigation, asking anyone who saw the attack to call 101.
A spokesman for Gloucestershire Constabulary said: "The group are described as including both boys and girls, all white and aged around 14 to 15 years old."
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