The controversy stems from Cee Lo's recent announcement that he'll be replacing the pet cat who accompanied him during Season 1 with a Moluccan cockatoo named Lady.
But the move is causing some concern among bird activists -- because Moluccan cockatoos are endangered ... and are known to be extremely TERRIBLE pets.
TMZ spoke with Monica Engrebretson -- a rep for the animal rights org. Born Free USA ... who also happens to be an expert in all things related to the Moluccan cockatoo. She's also a HUGE Cee Lo fan.
Monica tells us ... "Moluccan cockatoos are beautiful, intelligent animals but they are very challenging to care for especially in the long term, and are prone to considerable welfare problems." She adds, "Many Moluccan cockatoos develop self-destructive behavior including feather plucking and self mutilation not known to occur in the wild."
"Many parrot rescues are already filled to capacity with moluccans and other large parrots. I encourage “The Voice” to reconsider featuring a moluccan cockatoo or any bird as a 'pet' on the show and instead promote a more appropriate companion animal such as a dog. Even better, they could feature a rescue dog or a dog in need of a home."
She adds, "I want to add that I am a huge fan of Cee-Lo and I am glad that he appears to be an animal lover."
We reached out to Cee-Lo -- so far, no word back.