A "chicken tractor" is a mobile pen for chickens. I think the term was coined by Bill Mollison, the father of permaculture.  Birds fill an important ecological niche in the wild, but in agriculture we need to honor arbitrary things like "property lines" which creates a couple of problems: their poop is too 'hot' and will burn plants and eventually make an area into a dead zone, and a concentration of yummy chicken that can't fly away to the other side of the river attracts predators. The solution is to have a pen that moves and only deposit 24 hours worth of manure at a time in one spot. Pasture recovers fairly quickly from that and its re-growth is greener, healthier, and seems to hold water better.
this is mid-may after one pass with a chicken tractor. can you see the 10' swath of emerald highway on the left?
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this is mid-april after 2 passes with chickens on both sides of this row.

We started with the design created by Joel Salatin 15+ years ago at Polyface farm in Virginia. 10' wide x 12' long x 2' tall, this pen comfortably holds 75-80 full size cornish cross. 100 is too many. His are made with pressure treated wood and corrugated aluminum roofing, but you can't find corrugated aluminum so we used galvanized which is HEAVY. Mayb we put a little too much wood in for bracing, but the resulting pen is very heavy and not very predator proof. Racoons once they get interested will move any size wood you put down to cover holes aside and skunks and weasels will dig under. Salatin just puts leg snares all around his pens when this happens but these are not legal in CA and we'd also like to coexist with our wildlife whenever possible. We saw a farm in Oregon's blog with a picture of their tractor with an electric fence wire around the bottom, so this is what we mostly do now. It hasn't kept the weasel out, but is fairly effective against everything else.


Salatin also has a zillion acres on his one farm and he never has to move his pens to another site, but we were looking for something that would work in CA on our smaller plots of land, ie. something that could be taken apart, loaded on a truck, and reassembled at a second site being leased.  I also wanted a tractor that could be moved by any size farmer, not just a 6' tall strong man only like the Salatin design. Lots of women in farming these days.  Pens that can move to other locations is also going to help with my weasel problem, I'm just moving the whole operation to a site without a weasel on it. I worked with SC Barns' Ryan G. and grabbed some ideas from my chicken farming mentor Sarah B. (Wild Chick Farm up in Humboldt Co.) and between us we came up with what we have now: the same 10x12x2 shape that seems magical for creating the right microclimate (cozy when its cold, cool when its hot) but out of galvanized pipe and corrugated roofing that is held together with standard panel clamps. It has 2 strong no-flat wheels that drop down and a trailer hitch in front so the whole unit can be pulled with a trailer dolly.