Eighteen birds have been beheaded, a frog speared, fish poisoned and 130 windows smashed at a Manchester park.
The vandals broke into the aviary in Wythenshawe Park between 1630 BST on Saturday and 1000 BST on Sunday.
After beheading the birds, they threw the carcasses around. They then filled a pond with fertiliser, killing the carp. They also chopped down trees.
Greater Manchester Police said they believed more than £10,000 of damage had been caused.
Pc Terina Wild, of Greater Manchester Police, said: "The level of vandalism and cruelty shown by the offenders is shocking and distressing.
"This is an extremely serious offence and I would ask the community to help me identify the culprits.
"Did someone you know come home with filthy clothing or were they acting strangely?
"If you know who is responsible, but didn't get involved at the time, please come forward."
She added: "This is an appalling crime which has affected a community facility and the offenders need to be brought to justice."
Councillor Mike Amesbury, Manchester City Council's executive member for culture and leisure, said: "This is a sickening act of cruelty and vandalism, causing significant damage to a centre which has been enjoyed by generations of people.
"I know that everything possible is being done to catch those responsible for this appalling crime and bring them to justice."