I've heard of stories of people feeding dangerous foods... such as avocado, apple seeds, chocolate, coffee, etc. The coffee one with the cockatoo I thought was pretty darn stupid! All it required was common sense, as the coffee was causing the 'too to become aggressive and unhandable. Remove the coffee, and the bird, who could not be handled in years was suddenly very sweet and gentle again!
I think that as long as birds are not sensitive to the 'bad' foods, and they are constantly fed it, then the toxin will build up over a period of time and eventually kill them.
Feeding dog and cat food is also highly dangerous. I did hear of one cockatoo that went into kidney failure after eating dog food, and I don't think the bird was eating it for very long at all... and yet, I've heard of others eating cat food for months if not years.
Spinach, kale and parsley are some other dangerous foods that if fed as the sole source or main source of fresh foods, it will kill your bird, given time. Fed in small, minute amounts, it can be a healthy addition to the diet.
And of course beans... especially red kidney beans. Toxic if fed raw or undercooked or cooked improperly.
Phaseolus vulgaris - Wikipedia, the free encyclopedia
I wonder if avocados are considered toxic because there is a higher possibility of the toxins leaching into the meat than say that of other fruit... apples, cherries, apricots, peaches, etc. If that's the case, then it would make sense why avocados are said to be avoided while others with toxic pits are considered safe.