I would be careful about getting into supplements game--it's easy to go overboard and end up having negative effects on the birds (as well as wasting money). And then if anything goes wrong with one you have more things to try to eliminate as suspects to the problem.
Do you garden? Whether you do or don't you might consider raising a few veggies just for the chickens, who will greatly appreciate it. Swiss chard is a leafy green that produces all spring, summer, and fall--just harvest the outer leaves and throw to the chickens. They also appreciate tomatoes, cucumbers, squash, melons, kale (and any yummy green cabbage caterpillars on them) and every green that humans eat. Plus they eat weeds like dandelion leaves, dock, purslane, or grasses/clovers from your unsprayed lawn.
