If you can pick up greens at the grocery store, it would really be good. I grow and buy turnip greens because they are really high in calcium, they are cheap, and the birds love them. I also agree- cut back on the corn and replace it with other whole grains like oats and millet and throw in some layer crumble to round it out nutritionally. Don't forget grit or a bowl of play sand will even work.

I occasionally also give my geese spinach leaves that I get from the grocery store. Bought some seeds so that I could grow my own!