A zoo killed 11 of their exotic birds by accident after keepers buried poison in their enclosure to eliminate rats.
Poisonous pellets aimed at killing vermin were buried around the animal enclosure after five of the brightly-coloured birds were killed by rats when they landed on the ground.
But the pellets were brought up to ground level by the rats and eaten, instead, by the unsuspecting rainbow lorikeets.
The Australian-native birds were decimated while still in their enclosure last week when their numbers fell from 26 last November to just ten.
Drusillas Park in East Sussex was forced to call in pest controllers after five lorikeets died between November and January after being attacked by rats, and decided to bury rat poison underground in the enclosure.
But an "unprecedented situation" meant rats increased in number and became bolder, and the rodents dropped the poison above ground.
Laurence Smith, managing director of Drusillas, said: "We were losing lorikeets at a rate of about one a week in the enclosure so we got together and decided we had to do something as the rats were becoming more bolder.
"We therefore employed a professional pest control company who placed block bait deep underground where we knew the rats were nesting.
"Very unfortunately the rats subsequently dropped some residual bait above ground level which some of the birds ingested.
"As a result we lost a further eleven birds making a total loss of 16. As soon as this happened the remaining birds were shut away to ensure no further casualties."
An animal rights group protested outside the zoo on Sunday, claiming that the zoo's pest policy caused 22 animals to die including a red panda and a meerkat, which the zoo denies.
A spokeswoman said: "We can categorically confirm that no other animals in the park have been affected by poison, this is completely untrue.
"We did lose a meerkat three weeks ago and our vets have confirmed that it was natural causes, most probably a blood clot, and it was definitely not poisoning.
"This was a very unfortunate accident that has upset the entire zoo team and it has been made worse by statements about other animals that are malicious and completely untrue."