Although chickens generally do not operate farm equipment, "chicken tractors" have become popular among organic gardeners and practitioners of permaculture, the international movement promoting ecological, community-based agriculture. A chicken tractor is any number of movable enclosures designed to protect and place chickens where their natural activities, such as eating and creating manure, benefit the garden - and themselves.
At Pacific's B Street Farm, chickens are used in a multi-faceted way. The wheel-equipped chicken enclosure contains nesting space and allows the poultry plenty of air and sun while shielding them from predators. The chickens in turn peck away at bugs, unwanted grass and weeds, while fertilizing the soil. An added benefit for human keepers: eggs. When they have worked over a site sufficiently, the chickens are moved to another location.
According to Andy Lee, in the book "Chicken Tractor: The Permaculture Guide to Happy Hens and Healthy Soil," the term was coined by permaculture co-creator Bill Mollison. Several books and Web sites detail how to build a chicken tractor, including: http://gladstone.uoregon.edu/~nfantasi/Chicken%20Tractors.htm; and http://www2.gsu.edu/~biojdsx/fowl/tractor.htm.