They cover sticky liquids with dirt I think to keep from getting stuck in it. Some believe they cover it to hide it from other ants. Sometimes they mix dirt with liquids so they're able to easily carry them back to their nest.
Watch out with bird seed, while a lot of it is probably too large for them, I hear it can also be bad for them. I have given mine some in the past, but they rarely seem interested. In my experience, they are most interested in grass seed, sunflower seeds, and poppy seeds.
They usually love most any insects you give them. While the nanitics seem to be pretty picky, from what I've seen, the more there are, and the older they get, the more normal their eating habits start to become. Pretty soon you will drop any insect in there with it's juicy insides exposed, and they will be all over it.
As far as overfeeding, I don't think that is an issue. Ants know exactly how much they need to eat. They can however, eat too much protein relative to the amount of sugar, but I think you would have to be exclusively feeding them something made with very high levels of protein in order to create that situation.