Yes, avocado is toxic to all birds and should be avoided. Yes, technically, macaws eat them in the wild, and it's debatable about what part of the fruit or seed is toxic, but why bother experimenting? Other foods to avoid include caffeine, dairy, chocolate, and alcohol. Although, it might be pretty funny to see a drunk guinea. I've only ever gotten the chickens buzzed on fermented watermelon they drug out from under the coop.