Some people raise their skepticism to the level of a religion. There was an African grey parrot who died in 2007, about age 30, who was the subject of almost 30 years of research--Alex. Alex was taught to say words in English, and to use his vocabulary in the research project. He was not given "treats" or any other rewards for speaking. The woman who set up the project went to a pet shop, but did not look at the birds for sale; rather, she just told the clerk to go get her one. She didn't want anyone to be able to say she had attempted to get a "clever" bird.
Alex was capable of recognizing shapes, colors and materials. Shown a green plastic cup and a green plastic key (a child's toy) and asked what was different, Alex would clearly say "shape." Shown two objects of the same size and shape, and asked the difference, he might say "wood" to distinguish it from a plastic object. (I don't know if he ever learned to say plastic."") If he didn't want to begin the day immediately with the trials, he would often say "want grape," meaning he was hungry ("grape" was his omnibus term for food). When other birds were introduced into the study, one of his dominance traits was to criticize the quality of the other birds' speech.
You can read the Wikipedia article about Alex by clicking here.
You can read Alex's obituary in The New York Times by clicking here.
You can read about Alex in a British Library article by clicking here.
Dr. Pepperberg's methodology was considered sound by other animal behaviorists.
Yet the religiously skeptical would have us believe that a high-level primate was just waving her hands around to please the humans. A bonobo called Kanzi was a part of a methodologically careful study in which he pushed touch keys to produce a spoken word, and did so in response to human speech. You can read about Kanzi by clicking here.
That particular machine-method has been in use for decades to assure that the test subjects are not just waving their hands around to please the humans. With Kanzi, the reseachers wore masks so that Kanzi would not be said to have been reacting to their eye movements.
Really, that sort of skepticism assumes that the humans doing the research are stupid and gullible. I highly recommend reading this Wikipedia article about the chimpanzee Washoe, the first great ape taught to use ASL.
The first time she saw ducks, she signed "water birds" to her human companion, although they had never discussed water fowl. I found this line in the article particularly telling:
When new students came to work with Washoe, she would slow down her rate of signing for novice speakers of sign language, which had a humbling effect on many of them.
But what the hell do I know--they're just dumb animals, right?