I think the important thing to remember is the phrase should be 'african grey *CAN* be very good talkers'. Jacko is twelve and speaks rarely (she says her name, hello and what are you doing?---most of it sounds like jumbled gibberish like a baby though).
Greys are extremely intelligent regardless and are quick to pick up on new behaviours but it really depends on how much they trust you/feel like it in regards to tricks. Sort of like cats. They also probably wouldn't be big on physical things like lying on their backs or being flipped upside down...
For example, Jacko can and does know how to recall from like 12 feet away....*if* she feels like it. More often she'll play dumb and make me come closer and closer until she politely steps up onto my hand like 'ta DA! I did it, now where's my walnut?'
