It's difficult to define it all and I think that's fine since there are a lot of things to take into account.
A chicken (today's domesticated types) can only get about 5-15% of what it needs from foraging. But it depends on what grasses and forbs are available in addition to the protein load (spiders, insects, worms, etc.). Some land has a lot; some has a little. It depends on the soil and the climate and past care of the land. Our range and pasture and meadow and yard and weed patch and forest area and pond areas are all different. But I don't think any of it is rich enough to give my chickens more than 5% of what they need on any given day.
Also, if the land is "poor" and the chickens de-nude it of vegetation, then they are not free ranging by my definition no matter how large (5 acres) or how few fences they have.
We have to move our coops around to get them to use just the sections allotted to them, which might be 3 acres with them sneaking into other areas at times. They don't seem to travel much more than about 50 yards from their coop. Some of it is a trees/bushes (cover) issue; if there's no cover, then they don't go as far. (but we always give them some cover very close by because they just feel more secure if they have some.) Some of it is just what their actual range is, you know, biologically what their territory is. Sometimes it's a matter of the tempting taste of grasshoppers that just keeps them going further and further away from their coop until the rooster realizes it's time for a drink and bring them all back for water.
I saw one of the older hens, with a good memory, running from over next to the garden all the way through our backyard and over to the pond area this morning to muck about in the mud and under leaves, but last summer's chicks have no idea where she's going or why and won't follow. However, if the coop gets moved toward the pond this spring/summer, they will figure it out. The old hen's range is larger than the younger hens'. Is one free ranging and the others not because they certainly don't go as far as she does and she definitely gets a different diet than pals. My Marans are almost feeder parasites in comparison to some of my other chickens. So, while the Marans can travel as far as they want, they mostly stick to about 10 yards from the coop. (Sillies.)
About the richness of the pasture or foraging area ... for us, many, many of the bugs in an area are cleaned up in just a week. Of course, it does depend on the range, meadow, pasture, yard. This initially surprised me and I was a little miffed because I thought that they would get to eat grasshoppers all summer, but once they're eaten up, they're gone. Mostly.
Grass fed ... another term that can be misunderstood although it is very straightforward from my perspective. Are the chickens let out each day for a couple of hours to eat some grass? They eat their own chicken feed also because that's can't survive on grass only. However, that grass, if they can get it, makes a proportionally larger difference in the eggs that one would expect. I would pick a grass fed hen's egg over a "free range" labeled egg at the store. Will "grass fed" be misused as a term. Yes, but then, we can just ask questions of the seller if we're actually going to buy the "grass fed" eggs and make our buying decisions based on that. Since we personally eat our own eggs and grow a little bit of wheat grass in the winter and free range in the summer, our grass fed eggs are good enough for us.
And to me, a pasture is a field that has been planted with pasture grasses for cattle. It's not a wild untouched field; that's range. You know, Home, home on the range ... where the deer and the antelope play and also where cattle are ranging near the sign that says, "Open range. Be prepared to stop for cattle on road" and the road is gravel. A pasture has richer grass than the range because it's been planted, but the range sometimes has a stream running through it and some trees now and then or bushes. But a pasture can be different in different parts of the world. So ... regulating the term pasture ... wow, that would be difficult, I'd think. But you'd think that any planted grassy area (backyard?) could conceivably be called pasture. It's about what they're eating, I think, when people use the term "pastured," not about size. Isn't it about the omega 3 vs omega 6 ratio in the eggs the reason that people want a "pastured egg" these days? You know, Weston Price and Paleo stuff.
Anyway, how did I get started on this? I think I just changed my mind three times while blathering on about this just now. I am not going to worry about anyone wanting to call their chickens pastured or free ranged or cooped up or corn fed or house chickens or heritage or Americanas (spelling that way on purpose)--you can call them anything you want and I won't call you on it. There are better terms than others, but I'm just going to be happy that I've found someone to talk about chickens with.
