Wow. My hat's off to you. That's about the deepest expert description I've ever heard in the chicken realm.
At the same time, I feel compelled to say, "Kids, don't try this at home!" Learning how to notice and interpret all these signs is valuable to any poultrykeeper, but it must have taken you a while to have picked all this up!
You're right about the grain being the first thing exhausted on range. Back in the old days they did a lot of research on this. First, calories are depleted, then (probably) protein, followed by vitamins in the form of palatable green forage. So the first thing to supplement is calories, usually in the form of grain. (Plus calcium except on calcium-rich soils.)
Robert
For most I would also say do not do this at home as acreage and predator management requirements exceeds the typical backyard capacity. Still some useful information providing understanding.
I have been seriously into chickens since the 1970's. Also generally had multiple flocks at multiple locations at any given time. Most of the flocks were based on game chickens / fighting chickens occasionally with some sort of pseudo-production breed (i.e. White Plymouth Rock, Barred Plymouth Rock, and Rhode Island Reds) included as hens. The games where generally organized as harems on what we called walks. The walks where usually established in barnyard settings where all we wanted to keep was stags / cockerels hatched before June 1 and the brood cock. The males where generally harvested about this time of year. The balance where then consumed by us or owner of the barnyard. The flocks were not fed directly and many were not in the company of any livestock that where fed. We also had walks that were basically doing all their business well away from any buildings and where alone fence rows. Roosting, nesting and chick rearing was all in those fence rows in a near feral state. We had some walks that had birds year round and they generally had more hens and younger stags to overwinter on. It was those that provided some really interesting observations. One walk then was based at home one of the hens liked nest in my bedroom as window was always open allowing for really close observations of broody behavior. Hen brooding clutch within a couple feet of your head makes for interesting sounds at night. When younger my observations not based on science.
These days I manage for at times as many as 4 discrete flocks on 6 acres of my own 18 acres with some of the flocks very much foraging much larger areas off my property. Neighbors were consulted first. One of the flocks has been based around the house and actually roosted on the front porch within a few feet of my bedroom window. I am now a trained animal scientist with a strong nutritional background. When I get days off they are often used to study the free-ranging flocks. On multiple occasions I have set an alarm to go of at 15-minute intervals around the clock to go out and see where the birds where and what they were doing. Season, weather, wind direction, time of day, content of feeding station, were just some of the factors that influenced ranging habits. I have also resorted to making so roosts very easy for me to walk up to each night so I can rapidly count birds and access from feel to estimate fullness. Birds also tolerate touching breast well but that is slower and in my setting it is not a very sensitive measure of nutritional status.
This stuff is down right fun to watch!