Dolphins, Apes, Pigs, Whales, and.... Parrots!

Whales and Apes, Whales and Apes, Whales and Apes.

I've noticed in recent readings that many of the authors cite examples of animals with high cognitive ability; the most commonly cited demonstration of high cognitive ability is the ability to recognize oneself in a mirror. People overwhelmingly use the examples of dolphins, apes, and whales; sometimes this list includes pigs. I would agree that these animals certainly display high intelligence and cognitive ability. However, I have noticed that one particularly bright animal is left out of the mix almost every time; I have not read a scholarly defense of animal sentience that uses the example of the parrot.

Many kinds of parrots are incredibly intelligent. There are many documented cases of parrot intelligence. The most famous of these is the African grey parrot, Alex. Alex is credited with several cognitive feats - among them are the following:
- Seeing himself in a mirror and, being unfamiliar with the color grey, asking "what color me?". When he was told that he was grey, he was thereafter able to identify the color grey on other objects.
- Alex also became aware of the concept on none before being taught about zero - he was asked how many red objects there were in a mess of green, blue, yellow, and orange objects. Seeing none, he just shook his head. Note that the concept of zero is relatively new to human knowledge.

Also, one African grey parrot was taught the concepts of bigger and smaller, and, using that knowledge, said "I want bigger" when his owners tried to put him in his cage; he refused to sleep in his cage until his owners bought a sufficiently large enough cage. It turns out that the parrot would stand for nothing less than his own full room. Fortunately, the owners obliged.

So, my question, related to this, is why is it that people tend to exclude parrots as examples of intelligent animal species? Is there something that is gained or lost from doing so?