My birds free range, in the 9 years I have had my own birds and the additional 10 years of growing up with birds, I have never seen disease in any of them. That is hundreds maybe thousands of free ranging birds all intermingling together. I understand the concern for disease, but free ranging birds tend to be healthier and much more resistant.
In regards to the feed, because they free range they do not require the same amount of feed as non-free ranging birds do; however, my Turkey's refuse to eat game bird food, and will only eat layer, although I buy organic layer and not the stuff made by the other companies. That might have something to do with it.
I was wondering in regards with the pheasants, if they would fly away. The wild ones growing up weren't much for fliers, but they would glide around.