Since my post about chickens which mentioned the Linda Woodrow inspired chook dome. I have noticed that people are searching in google for information about chicken tractors. This post provides a few more links to information on this subject.
The Chicken Tractor gallery has over 140 pictures with many different types shown. As the construction skills required are very basic, a good picture will often be enough for you to build your own using materials to hand.
Commercial Chicken Tractors
before Permaculture they were generally just known as moveable coops or arks)