I free range; they can choose where they go. Range areas aren't bare except right in front of the coop where snows melt first and chickens scratch up the entire root system in spring, before the rest of the ground is bare. Also, it's built right on the edge of an area with deep leaf cover and a more brushy foliage spread, so any grass that gets destroyed by anything very quickly is swallowed by the forest. For me, building a moveable coop would be limited good at best because there is very little land cleared enough to move pens and stuff onto, and the bit I do have cleared (except for where the coop currently is) is unsafe due to proximity to known bear haunts. Yes, a hundred feet is a small safeguard, but it's probably saved my flock before: there's a rather bark-y dog and and two electric fences between my chickens and the woods this way.
Even, say, that all didn't apply, and I split my flock up into pens small enough to move around via tractor, which in itself would be five times the work, I would have a wicked hard time dealing with winter. I'd have to basically turn each one into a permanent structure to get it to bear up under snow and wind load. I'd have to make it way larger than I'd otherwise need for that few birds and I'd need to do that over multiple pens, which in the end, results in a huge amount of space lost to allow appropriate space for bickering birds to get out of each other's way. One 12 x 12 I can dump all my birds into turns out to be the most space efficient, least costly, and snuggest way I've found yet. I've kept birds in tight housing over winter, and trust me, it's seventeen buckets of no fun. Heavy on cleaning, heavy on health issues, heavy on work. The fewer buckets I have to break ice on in winter, the happier I am. I'm already going to be carrying out 8 gallons of water per day, easy. I don't want to have to do any more. I'm not familiar with the climate of Spain, but I'd bet you my next paycheck that it isn't anything like what I have.