I've read the same as Loko concerning mycotoxin. It was in regards to meerkats at a zoo rolling their grapes in clay before eating them. Clay is used in naturopathy to make poisons and toxins inert. The purpose of ingestion of soil and minerals in wild parrots is believed by many researchers to nullify the many toxins in the seeds and nuts which comprise the majority of a parrot's diet. Plants try to deter predation of their unripe seeds and nuts by using some sort of physical deterrent (hard shell, spikes) or chemical deterrent (caustic, toxic, or bitterness). When their seeds or nuts are ripe, many plants encourage animals to eat their seeds so they'll spread them in their feces by offering fragrant, sweet, colourful fruit, or by losing or weakening the physical or chemical defences that they were using to protect the unripe seeds. Parrots feed on unripe seeds and nuts, using their powerful beaks to crush the hardest of shells (i.e., macaws, cockatoos), and are able to withstand the toxins in the unripe seeds/protective outer flesh that would cause other animals to become violently ill or leave them with chemical burns. Obviously I'm not suggesting to give captive birds such foods, as they don't have access to the same nullifying compounds that wild parrots have access to.
Same as Dinosrawr was saying. Plus, you've gotta remember that parrots can eat the hottest of peppers without getting gut rot, further supporting not just the use of detoxifying and nullifying compounds, but also physiology.
Hey, Billdore, as far as pesticide concerns go, I'd start buying from health food stores. At my local one at least, all their produce is certified organic, so you don't have to worry about transference.