I bought a big bag of black Sunflower seeds recently. It claims to be fortified with Vitamin A & D and possibly other vitamins & minerals; I forget the exact claim. I saw this on a lot of other bird feeder type products, but most of those are a mix of items. I expected there would be something in the bag besides the sunflower seeds to somehow hold bits of nutritional goodies. Instead, the bag is 100% pure black sunflower seeds. So I started wondering - how does the "manufacturer" fortify Sunflower seeds like this? Some sort of liquid sprayed on to the seeds? But - the critters generally don't eat the shells. Still perplexed about this. Certainly sounds like a good idea, but how does it work?