Chicken Tractors (AKA Chicken Arks) gives chickens the advantage of foraging, nesting, and sheltering in a protected space. They receive a fresh place to graze, eliminate/fertilize, and contribute to the overall health of the grass/pasture, all the while giving them adequate sunlignt, and excercise - mimicking the natural brooding pattern of the bird found in nature.
The tractors became popularized by the legendary Virginia farmer and lecturer, Joel Salatin, whose whole systems approach to animal husbandry has garnered many followers.
Here is a link to Joel Salatin's website:
And a great Chicken Tractor Gallery site to inspire your design:
And a great, inexpensive eBook on the Design and Use of a Chicken Tractor: