From what I know by researching the subject there are no necessary nutrients for birds in any onion, garlic or leeks. I do think it takes more than just a little ingested to kill a parrot but why take chances?
When introducing new herbs or foods to any parrot I always suggest we error on the side of caution. Although some parrot species may be able to consume small amounts of fresh or powdered garlic without ill effects, I think running the risk of causing hemolytic anemia is too great a danger, especially when there are safer, gentler natural alternatives. Years ago when I interviewed Dr Greg Harrison (Harrison’s Bird Diets) he explained that he and many of his colleagues would not do feeding trials on birds to test a food, chemical or substance they knew or suspected would cause harm. I applaud Dr Harrison’s attitude as I think we need to replace traditional feeding trials with a more humane, long term system, of feeding studies.
Avian veterinarians have repeatedly stated that malnutrition is the leading cause of illness, disease and early death of birds. If nutrients are missing from a bird’s diet, this results in either malnutrition or undernutrition.Patricia Macwhirter DVM, (in Avian Medicine: Principles and Application) malnutrition or undernutrition can cause a specific health related, or behavior related, problem or it can suppress a bird’s immune system. When the immune system has been suppressed the body becomes vulnerable to infection from a variety of pathogens: bacteria, fungus, virus or parasites.