There are several fruits that are poisonous to birds and should not be given to a bird to eat. It is fine to give birds a few grapes as long as you can remove the skin of the grape and make sure there are no seeds in the grape. The seeds of grapes can have trace amounts of cyanide in them that are harmful to the bird. Furthermore, the skin can contain traces of pesticide that can harm your bird.
This holds true for apples. You'll need to clean the apples, remove the skin and seeds before feeding them to your bird. Avocados are yet another favorite of birds, but the skin and seeds are dangerous to the bird. If you decide to feed your bird one of these fruits it's very important that it is cleaned properly and the skin and seed are not fed to your bird.
Cherries, peaches, apricots, and pears can also be given to a bird as long as the skin and seeds are removed. Never give your bird anything with seeds in them because this can kill the bird. Make sure that all skin and seeds is removed from all fruits before feeding them to your bird.