Baby birds have died and up to 60 finches and cockatiels have escaped after vandals smashed the doors of their hidden away home.
Dozens of frightened birds took flight after the padlock holding the door to their aviary in Cannizaro Park, Wimbledon Common, was broken, setting them free.
But many of the birds are known to have died as they are not used to fending for themselves in the wild, Friends of the Cannizaro have said.
Police are investigating after many of the winged inhabitants escaped.
Some were found in nearby garages but many have been lost or have died.
Some of the many finches and cockatiels returned to their damaged home frightened of the big outside world.
Friends of Cannizaro Park chairman Chris Mountford said: "We don’t know who did it - there was no animal liberation group.
"We think it was local thugs.
"The birds will die - they are seed eaters but there are no seeds around."
Mr Mountford said two cockatiel chicks and their mother have already died, but they have managed to save another six week old chick and a mother.
"It’s really sad," he added.
The birds were being housed in a temporary aviary while their home was undergoing a £10,000 refurbishment.
The newly painted aviary in the park in West Side Common is due to open next week, with the remaining birds set to be re-housed.
The ornate aviary near the entrance to Cannizaro Park is a well-known landmark for visitors wandering into the Grade II listed garden but ground staff had reported it was leaking water and was in need of a lick of paint.
Members of the friends group found the temporary aviary door smashed when they went to check on the birds one morning.
Mr Mountford said: "It is a wreck. "The doors had been forced open.
"It must have been frightening for them."
Mr Mountford is looking after some of the remaining birds in his own home while repairs are made.
A police spokesman said a fence surrounding the aviary was found to be secure and, coupled with some finches being reported sheltering in a nearby garage, police believe it is possible the some, or all of the birds may have been released into the surrounding area.
Anyone with information about the crime or the location of the birds should contact Merton police by calling 101 and quoting reference number 1404575/14.
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