what happens if parrot eats avocado? I know its bad but I don't know why?

Very likely nothing. The meat of the avocado is not poisonous to them, but the skin and the pit are toxic. However, a cracked pit can leach poisons into the flesh, as can bits of skin. Since it's a high fat food anyway (which they don't really need), it's better not to risk feeding it to them.

for a list of symptoms of persin poisoning, see http://en.wikipedia.org/wiki/Avocado#Toxicity_to_a...

Add: or just read hallowsweets answer, since she copied the relevant portion of the wikipedia article.

Funny, because I see birds of all sorts, including parrots eating avocados in the wild all the time.

There is documented evidence that animals such as cats, dogs, cattle, goats, rabbits, rats, birds, fish and horses can be severely harmed or even killed when they consume the avocado leaves, bark, skin, or pit.

The avocado fruit is poisonous to some birds. Avocado leaves contain a toxic fatty acid derivative known as persin, which in sufficient quantity can cause equine colic and, with lack of veterinary treatment, death.

The symptoms include gastrointestinal irritation, vomiting, diarrhea, respiratory distress, congestion, fluid accumulation around the tissues of the heart and even death.

Birds also seem to be particularly sensitive to this toxic compound. Negative effects in humans seem to be primarily in allergic individuals.

Avocado Bird

They can cause respiratory distress and congestive heart failure

the flesh has something it that's poisonous to birds.