Sick as a parrot

Question
Where does the expession "sick as a parrot" come from?

Answer
Very sorry for the slow reply!  I haven't been getting the notices that I have a question waiting.

I imagine that the expression "sick as a parrot" stems from how parrots deal with illness.  Generally they show no signs of being ill at all until they're extremely so -- so you rarely see a sick parrot until they're close to death.  But at that point, they're very obviously sick, and I suppose the expression "sick as a parrot" was inevitable!

"Sick as a dog" is a mystery to me, though!

- Lara