it should be mentioned that there
is a difference between learning words and learning a language. I'm not aware of a single case of and animal learning a human language.
a parrot or ape may learn different words, and and some aspect of their meaning: "water" (they bring me water), "outside" (they take me outside), "hug" (they put their arms around me), "red square", etc. etc.
but "say, John, could you please take me outside so I can have me a drink of water," is miles away from "outside! water! outside! john! water!"— which is the best an animal can achieve, and after endless hours of training.
human languages are a way of communication that has evolved with humans and is deeply rooted in our brain (or not... is the jury in on this yet or are they still fighting? some of my books are a few years old and I can't keep track of everything... stupid progress

).
a parrot's brain is perfectly designed to communicate with other parrots, not with humans (useless info: dogs are one of the only species that instinctively understand human communication— facial expressions, tone of voice, etc. having evolved alongside humans, they are also better at expressing themselves— they make human-like facial expressions, and howl in a way that resembles a human's).
I haven't read about n'kisi, but if this bird can truly understand any aspect of the subtleties of human communication (jokes, subtext, syntax), then it is truly an exception and a freakishly smart for a bird.
I'm not saying these animals aren't smart, when I see apes and parrots and elephants at the zoo (or even my cat), it always shocks me how you can stare into their eyes and tell there really is
someone in there; it's not just some furry machine that eats. it just happens that they haven't evolved or have a need for language.