I don't have experience with capons, but I raised twenty five Cornish crosses last year. We started our birds on starter for the first week and then we switched them over to Purina's Show Broiler feed which has a high protein content and gets them nice and meaty. I've also heard that mixing carrots and vegetable oil with their feed helps too. You can also use bacon fat, but since we don't eat pork, I haven't tried this. In fact, I've even heard of people feeding their birds with ground beef once in a while, so you are pretty much limited to anything you can think of. Another thing you can do is raise mealworms and feed that to them in addition to their feed.