The basic idea for keeping broiler chickens in movable pasture pens comes from Joel Salatin, who raises thousands of birds that way each year in Virginia and has written numerous books on the subject. The idea is to keep the birds in a safe place, but outside in the fresh air, where they have plenty of good green stuff and bugs in their diet. By moving the pens each day, the manure never gets built up too much in one place --- and the birds have a constant source of fresh green stuff.
Our first year here, I built a few crude pasture pens and basically moved them around in the yard. There was little rhyme or reason to how I moved them or where. I also experimented with various materials and styles for building the pens, and wasted a lot of money (which we dubbed "tuition money") in the process. But I took good notes, and improved the pens each year.
Then, last year, came a big breakthrough. I began planting a new vineyard at the north end of the property, and fenced it off securely from the sheep. The vineyard consists of four very long lines of wire trellis, stretching from north to south. The first line, on which I planted approximately 35 vines last year, is about ten feet from the property's western perimeter fence. The next lines (one of which I planted last year, one of which I planted this year, and one which will be planted next year) are each eight feet further east of the previous, and there is an eight foot gap between the final line and the sheep pasture. All five of those 8+ foot aisles are filled with a wide variety of weeds and clover.
Rather than mowing down all that green stuff, I decided to let the chickens do the work. We put three pasture pens, each of which is approximately six feet wide and eight feet long, staggered on various aisles of the vineyard. Two of those pens contain laying hens (to keep them out of our fruiting brambles and Mrs. Yeoman Farmer's garden for the summer), and one contains a mix of broiler chicks and pullet chicks (next year's laying hens). Each day, the kids and I gather eggs from the pens with laying hens, move all three pens one length (8 ft) down the aisle, give the hens some layer ration and the growing birds some broiler ration, and ensure that their waterers are full.
As the pen begins moving, the birds scramble to snap up all the crickets and grasshoppers that are stirred up in the fresh weeds. They then begin picking at the green stuff itself, sometimes even ignoring the supplemental grain I've just put down for them. By the time I come out the next day, they've completely mowed down the weeds and given the vineyard floor a nice layer of fertilizer. This picture shows the amazing degree to which a pen of birds can mow down weeds --- notice the contrast between the aisle behind this pen and the height of the weeds in the adjoining aisles. Before the pen went through, the whole vineyard had weeds that high. (The blue grow tubes are where the grape vines are growing --- the tubes act as a greenhouse, and support the vines to help get them up on the trellis.)
This photograph gives a wider perspective of the vineyard, showing a couple of different pens moving in different directions.
I'm not sure exactly how long each trellis line is, but it's somewhere in the neighborhood of 200 feet. At 8 feet per day, it takes about 25 days for a pen to make it from one end to the other. There is a generous amount of open space at both the north and south end of the vineyard, to allow the pen to be dragged sideways and started down a new aisle. Once all five aisles have had a pen go over them once, we send the pen down whichever aisle's weeds have grown back the most.
It's a beautiful system: the birds are confined in a safe and manageable space, they get fresh air, fresh greens, and plenty of bugs in their diet, and are moved off their droppings. The vineyard gets mowed and fertilized, and a significant number of bugs are removed. It's "ecology" in the truest sense of the word, and I highly recommend it for farmsteads everywhere.