Here are some basics about chickens' space needs:
The Usual Guidelines
For each adult, standard-sized hen you need:
- 4 square feet in the coop (.37 square meters)
- 10 square feet in the run (.93 square meters),
- 1 linear foot of roost (.3 meters),
- 1/4 of a nest box,
- And 1 square foot (.09 square meters) of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
12 hens
- 48 square feet in the coop. 6'x8' is more practical than 4'x12' since a long, skinny coop like that would be difficult to work inside.
- 12 feet of roost
- 120 square feet in the run. 10'x12' or 8'x15' -- 8'x16' means fewer odd cuts than either of those. 6'x20' is possible, especially if your run is an open-topped, fenced area instead of fully-enclosed with a solid and/or wire roof but risks social problems because subordinate hens need to be able to pass the dominant hens at a respectful distance.
- 12 square feet of ventilation.
- 3 nest boxes.
Note: These numbers are *guidelines*, not hard and fast rules.
This article is valuable reading on this subject.
A tractor that serves as coop and run combined for 12 birds *may* need less than the 168 square feet of run and coop added together if it's all one big space rather than a coop and an attached run on wheels. Or it may not.
Moving the tractor frequently solves some of the sanitation problems of chickens being kept in tight quarters, but it doesn't solve the social problems of chickens interacting in a close space. Especially because a tractor doesn't lend itself to having "clutter" in the run (useful thread on clutter
here).
The use of very large tractors is most common when raising broiler chickens with the main benefits of moving them frequently (even twice a day at the end of the production period), being feed savings through access to forage and improved sanitation by spreading the manure over a larger area. The social issues I mentioned don't arise because Cornish X "meaties" are essentially great big chicks in a great big brooder rather than adults in permanent housing.
Laying hens in a tractor-ish situation are more often given a coop-on-wheels such as was mentioned above as safe night roosting quarters and let out into a run made with easily moved electric poultry netting during the day.
Enclosed tractors work best on smooth, flat ground with little in the way of obstructions or irregularities. The sides don't seal down well on rough, irregular ground to keep the chickens in and the predators out.
I see @3KillerBs says almost the same thing, but must have typed faster than me:
