Generally speaking they can eat anything we can except avocado pits and peels, green/raw potatoes and peels, tomato leaves/stems, sugar/candy/chocolate, citrus (affects calcium absorption but not actually poisonous), dry beans, junk food (white bread, chips, fatty foods, etc), or anything moldy or rotten. You just don't want to go too overboard with treats because it is essentially adding or removing too much protein and calcium from their die. Most times I have a bowl of leftovers (like one time was meatballs and mashed potatoes) I throw it out for my birds. I throw them my carrot peels, celery scraps, apple tidbits. When the weather is hot I give them 1/4 of a cold watermelon, iceberg lettuce, a whole apple that's gone a bit soft, or things like that. Just like with people, all good things in moderation!