Where did you get the "20% of their diet" statistic? Not trying to cause issues, I would like to know your source and read the study.
I think that where you live has an impact on how successful chickens will be when foraging for food. For instance, there is a lot more available to my chickens here in Wisconsin, than there was in Wyoming.
In this case I don't have any set study. This is based on my own experience and acquaintances' experiences with free ranging and feeding free range birds.
And yes, you make a good point - while any breed besides perhaps a gamefowl won't likely obtain more than 20% of their diet via free ranging, how close they get to that number is going to depend somewhat on where they are located. I'd be willing to bet areas with good heavy foliage and tropical areas will have a lot more to offer than, say, a desert.
