[steveo]

but the size of their vocabulary and their ability to understance the context of what they are saying and when to say it.

Agree 100% on that. Our grey 'Bob' will say our cat's name when she comes into the room and then imitate her meow. If I close my office door he says 'goodbye' and then starts doing his telephone ring to get me to come back in.

I have no idea how many words and phrases he knows. Amazing creatures.

[zeugma]

There was a thread here a few months back (that I can't seem to locate) about a parrot that someone has been doing research on. It has a vocabulary of about a 4 year old, and can understand such concepts as "zero", and much other stuff.

Here's an external link for the story. Amazing!