Although I have talked about this before, I think it is important to mention again, as it is a common misunderstanding. So often I see people feeding both pellets and vitamins to their birds, trying to make sure their birds get everything they need to remain healthy. While the concern is well founded, I think this cure can do more harm than good.
When it is said that malnutrition is still the most common cause of death for captive parrots, it is important to note many things. One, parrots do not always die strictly of malnutrition, but of complications from malnutrition; a weaker immune system causing them to be more susceptible to bacteria, perhaps. Another thing that I think can be more important to note is that malnutrition simply means bad nutrition, not not enough nutrition. So both lack of vitamins, and toxicity of too much of a vitamin, hypervitaminosis, or a mix of the two, is one of the leading cause of death for captive parrots. It may seem odd to be talking about the dangers of too many vitamins; after all, we are always being lectured on how hard it is to get our parrots what they need, can we really give them too much? The answer, though a bit complicated, is yes.
If you are feeding only fresh foods, you have no danger of hypervitaminosis. This is because fresh foods, real veggies, fruits, and grains, etc, often contain precursors to the actual nutrient, so the body simply assimilates from the food what it needs, and leaves the rest. Vitamin A, for instance, is not a natural food of parrots, rather, they consume beta carotene, a precursor to vitamin A, from their food.
The problem arises when we start feeding synthetic vitamins. Synthetic vitamins are already in their final form, and so instead of the body being able to take what it needs and leave the rest, they have to use all that they are given. Vitamin A, using the same example, is one of the more common vitamin toxicities heard of; vitamin A is stored in the liver until it is needed, so too much of it can overpower the liver. Hypervitaminosis can show itself through all manner of symptoms including, but by no means limited to, plucking, poor feather growth/quality, excessive beak or nail growth, hyperactivity, aggression, listlissness, and an all manner of health issues including liver or kidney malfunction or failure, and unfortunately, death.
Now, however, things get more complicated. No one really knows for sure how much is the perfect amount of these synthetic vitamins for our parrots is correct. Each species has very different needs from all the others, and potentially different ways of precoessing the vitamins. Combine with this the fact that each individual parrot varies in how they handle synthetic vitamins and you have quite a puzzle! Going back to the vitamin A for another example, and in regards to parrots that are being fed an almost completely (80% pellets or more) diet, I have heard of an almost equal dying from lack of vitamin A as those dying from Vitamin A toxicity. This shows how very much individual parrots vary in their use of the vitamins, for some seem to be unable to process synthetic vitamins, and simply remove them from the body as effectively as they remove a toxin, while others become overwhelmed by storing all of the vitamins.
For these reasons, it is very often recommended now to feed a mostly fresh diet, with some pellets if you wish to include them or do not have enough time to make sure they are getting a fairly complete fresh diet, and time outside in the sunshine for vitamin D. Except for TOPs and FF, pellets are essentailly a filler, such as wheat, soy, or corn, with vitamins added. SO by including pellets, you are including daily vitamins. If you add more vitamins, you run the risk of hypervitaminosis.
Simply put, if you are feeding even a small amount of pellets, you should not add any vitamins unless a blood test shows your parrot is deficient, and then the vitamin should only be added for a certain time period until a re-check is completed, or as recommended by your avian veterinarian.