Cockatiel talking

Question
I have this delightful 5 year old male cockatiel that has a huge vocabulary, I think, for a bird.  Maybe 15 different words, three or four full sentences and then he makes up his own with the words he know, he blows his nose when I pick up a kleenix, coughs as I do, just a delightful mimic, when i take a drink he says "is that good? and then goes "ummmm", or when he sees the cat he says "did you call the kitty? and then whistles for her...... however... he seems to have learned nothing new in the last year or so... as far as words are concerned.  Is there a limit to how many they will learn? I have tried to teach him new ones but he sticks with what he knows and improvises them in different forms.  Thank you for your help....  

Answer
From what I've seen birds do have somewhat of a limit as to the number of words they use/know  (with the exception of amazons and african greys in particular).  I wouldn't be too concerned with it, he is probably starting to get more set in his ways and continues to do what he is comfortable with. But for a cockatiel- he says A LOT!!  That is very unusual for a cockatiel to be that proficient in the talking/mimicing department.  :)

-Maggie