Probiotics can be helpful in balancing the gut flora of both birds and mammals. While simply providing feed and water is a good thing, there may be times when a bird will benefit from an extra input of probiotics. Newly hatched, stressed in any way, or birds who don't have access to free range. Can be provided by artificial means, feeding fermented products including fermented feed, and even access to natural soils and compost. As other posters have stated, dairy is not a good option due to lactose intolerance.