BirdWatch Ireland has said it is "horrified" by what appears to have been a deliberate attempt to poison peregrine falcons nesting in Dalkey Quarry in Co Dublin.
The birds, a protected species, are nesting on a ledge in the quarry and have four chicks nearly ready to fledge.
The incident came to light last week after a walker noticed two tethered pigeons flapping near the nest.
The pigeons' wings had been clipped to stop them flying and they had what is described as a "wet substance" on their neck.
Peregrines eat live prey and would have been attracted to the pigeons by their wing flapping as they attempted to escape their tethers.
The pigeons have been submitted to the State laboratories for testing.
The National Parks and Wildlife Service said the falcons are facing "a significant threat" and "it is likely that further attempts may be made to interfere with the birds".
It is calling on the public who use the area to remain vigilant and report any similar incidents.
It also warned that people should keep children and pets away from suspect birds as these poisons are harmful to wildlife, domestic animals and humans.
Minister for Arts, Heritage and the Gaeltacht Jimmy Deenihan said: "It is not tolerable for these majestic and extraordinary birds of prey or any other wildlife to be persecuted or poisoned.
"It is illegal, but just as important; it harms our reputation as a country that values its wildlife.
"I would urge anyone to report such incidents to the National Parks and Wildlife Service in my Department. Harming our birds is not acceptable."