Oh, those ducks

How do you explain that chickens and birds are not ducks?
When Maxwell yelled 'quack, quack, quack' enthousiastically at all three (birds, chickens and ducks) I corrected him and said, no THAT's a bird, 'tweet tweet tweet' and no THAT's a chick, 'tok tok tok.'

He looked at me briefly and then continued saying 'quack, quack, quack' even more determined than before. Little guy, 1 year old, speaking only a few words and already he knows best. I think we've had our first verbal disagreement today :-).