Animal welfare officer Alison Sparkes, who was called by police, said: "It was a remarkable sight, I've never seen anything like it."
There is no evidence the birds were ill or poisoned before they hit the ground.
Ms Sparkes said: "Onlookers said they heard a whooshing sound and then the birds just hit the ground.
"They had fallen on to the ground in quite a small area, about 12ft (3.6m) in diameter.
"They appeared to be in good condition other than injuries that they appear to have suffered when they hit they ground.
Truly fucking bizarre!
