Commercial feeds are the best way to get a full spectrum of vitamins to birds and I always recommend that at least 90% of a bird's diet be a complete feed (pellet preferred, or crumble). But unfortunately, not all feedstores keep their feed in the kind of conditions to keep the vitamins effective - and sometimes the lack of turnaround in feed means we take a less-than-fresh bag home.
Some of the most important vitamins for avian health are oil based: A, D, E. They usually are able to stay pretty effective in a fresh bag of feed, stored in cool conditions, sold within 6 weeks, fed quickly. But that's not always the case of how feed goes. Add to that the fact that many people feed crumbles, which have a high surface to air ration, making it easier for the vitamins to degrade. So unfortunately the most necessary vitamins from feed sometimes degrade.
Because "vitamin packages" in feed are dry based, not oil, the actual absorbtion and effectiveness of the chemically produced A, D, and E often suffer. Those vitamins are really dependant upon oils and fats to be used by our birds. And sometimes birds, under stress, have an increased need for them.
So sometimes (particularly during stress or illness) we need to 'spot' supplement vitamins.
When you do so, using an oil-based source (a capsule, wheat germ oil used in moderation, etc) is the best way to do it.
Vitamin A is very important for avian respiratory health and ocular health. Unfortunately it's a very unstable vitamin and often the first to go. Vitamin D is a requirement for calcium absorbtion, but again because it's often exposed to light in our feeding conditions, and heat, etc it can sometimes be deficient. (Also to absorb calcium appropriately, birds require sunlight to transform the vitamin D into D3 which is the specific vitamin D for calcium absorbtion.) Vitamin E is immensely important to the function of the immune and neurological system. Again, it's oil based - it requires fats and oils to be absorbed correctly.
The thing with oil vitamins tho is, because the animal doesn't pass them as they do water-based vitamins, you must not overdose. For that reason using beta carotene is the best choice for vitamin A. But wheat germ oil, a non-chemically produced form of the vitamins, is also a good choice for the barn as it contains A, D, and E and is natural. I used this on my show flock and breeding birds at a rate of one ounce for about a gallon of feed - mixed well. I did it only when they were breeding, molting, or in the stress of showing. Otherwise I just do it once a month to make sure that the birds are keeping up with their necessary vitamins.
Hope this helps.