Two rescue birds have died after a hole was cut in the fence of their aviary in order to let them free.
The birds were two of three released from the Bournemouth Central Gardens Aviary in Dorset on Friday. The third bird returned of its own accord.
Freeing the birds in fact caused their deaths, police said.
Pc Steve Knight, of Dorset Police, said: "These birds are well looked after and are only able to survive because of the care taken of them."
He said: "Those responsible have not only committed criminal damage, but they have caused birds to die.
"The birds in the aviary are rescued birds and are unable to survive in the wild. Three birds were released from the aviary but two died before they could be recaptured.
"A third bird returned to the aviary on its own."
Dorset police were unable to confirm the species of the birds. The aviary is run by volunteer staff.
Pc Knight appealed to whoever was responsible "not to attempt to release the birds again because they will undoubtedly die".