George finding these sustainable breeds is a difficult thing-- because they are less popular and few have them, they get lost in the noise touting the popular breeds.
I started asking over 2 years ago about free ranging etc. GOt a beautiful reply from Joeseph ( Yellow House Farm) and only at the end did someone mention games; unfortunatte ly by then the dual pupose of the more popular breeds were fixed in my head; but as I said above, I never forgot what that man said-- while initally I didn't want to heat the advice , that changed to I just didn't know how to investigate it and find out more: all my questions were hitting closed doors. BUt I never forgot.
In my first batch of hatchery chicks I got a SSH, a little rooster that was slow to mature, and when he did he ruled the yard for a long time ( until the EE crosses grew up) and I appreciate his quick reaction time. Approachable to about 5 feet, then he was wary and could only catch him when he went to roost for the night. He was a tiny bird being hatchery, but I saw what looked like a fine example at a regional show and was stunned at the sheer size of that SSH ( SIlver Spangled Hamburg) -- and I often think of the possibility as a sustainable farm bird--.
I did not learn of the high protein requirement of the buckeye chicks until the day the chicks mailed-- was totally surprized as I have read the buckeye thread for several years, and though that might have been mentioned and went right over my head, it certainly was not a focus of dicssion like feather color is.
AM am still at trying to undestand what " sustainable" means . . . . and I suspect there are several models which reflect differing levels of self sufficency.