400 yards is less than 1/4 of a mile.
Oh, duh! I'm changing that in my post above. But I remember it feeling it like a mile when I had to race a 440 ... worst race ever.
But, back to the chickens ... it would have been great if you'd answered my questions: Do your chickens do this? Or where did you find this information? What kind of chickens are you talking about? Jungle Fowl?
I haven't run into the info anywhere that chickens range 400 yards in one direction ... well, except you saying "... a chicken in a true or un-limited [sic] free range situation can be expected to travel 400 yards in every direction...." And you won't even say whether you've seen chickens do this on a regular basis or not.
I can expect my chickens to do this based on what you're saying, but they are not! And they have wooded areas to do it in and they are not! If your conjecture is true, then I am perplexed that my chickens are not ever ranging that far.
If you care to reply, then it will save you time to know that I know what unfettered free range is and have it. Also, I know about Jungle Fowl and their behaviors and lifestyle. You should also know that we do not have anywhere near the predator load you describe in your post--maybe 0.00005% of a predator behind and in every tree or less.
If you pulled your numbers out of the air (as in not based on your experience, experiences of the ancients, or university studies), please say that so that I can stop wondering what is up with my chickens who won't range further than about 100 yards in one direction ... no matter where we place their house each summer.
I'd love to hear from anyone with actual experiences where their own chickens range 400 yards from their coop on a regular basis--even if this is an old thread when you read this.