What a great question. I was wondering the same thig the other night. I was wondering if it may be due to the fact that birds carry a lot of desieses(!) and therefore may be percived as 'sick' even though they may not show symptoms. Hamish
"Sick as a parrot" as a term for disappointment, especially in a football context, arose in the 70s, along with the opposite term for euphoria - "over the moon". This was about the same time as the Monty Python Dead Parrot sketch was televised and became famous, and I think there may have been a connection. Possibly, the phrase was originally "as sick as Monty Python's parrot", which then quickly got shortened.