First off, where are you located (what climate). It is a WHOLE lot more feasible to winter chickens in a tractor in, like, Virginia than in North Dakota
If you are closer to the North Dakota end of the spectrum then honestly I would say that a tractor would be far from your best choice as winter quarters.
The issues are:
1) Ventilation. Chickens deal with dry cold reasonably well (esp. if you pick fairly cold-hardy breeds, i.e. those with large body size and those with rose or cushion combs as opposed to great big single combs). What they don't deal well with at all is DAMP cold. You get respiratory disease and frostbite. Thus, you have to be able to leave the vents open virtually all winter (ok to close them if needed on occasional really frigid nights, but that's relaly about all you can do without courting a problematically damp coop).
The thing about a tractor is that it is a very very small volume of air, and thus a given amount of chicken poo and breath (which is where the dampness mostly comes from) create a lot more humidity in a 3-4' high tractor chamber than in a 6-8' high coop. So you need relatively MORE ventilation in a tractor than in a walk-in coop. But at the same time, the airflow can't be aimed at the chickens or they will get too cold in the draft and develop respiratoyr problems or frostbite. It is, at best, extremely difficult to get sufficient ventilation without draftiness in a small tractor.
2) Heat. A tiny coop such as a tractor also loses heat faster than an equally-constructed walk-in coop, for the same reason that a cookie coming out of the oven cools off faster than a cake from the same oven. Thus, although you need more ventilation in a tractor for dampness reasons, that extra ventilation will also be carrying off needed heat and it can be tough to keep a small coop warm enough.
3) Electricity. You can't run 'proper' electric service to a tractor, but if you live anywhere with 'real' winter you will be ever so much happier if you can plug in a heated waterer or waterer-base to keep the water from freezing. (The alternative is to carry water out there multiple times a day and expect the chickens to learn to drink while it's still liquid - you don't want to be putting out warm water, esp. not in a tractor, because of the humidity problem, so whatever you take out may freeze rather rapidly, depending on your climate). It is also AWFULLY nice during the short-day time of year to be able to flick a light switch when you have to do something out there in the dusk or dark. Some people run heavyduty extension cords, but you know that is really not the safest thing to do.
If you DO decide to go with a tractor for winter use anyhow, I would strongly, strongly suggest not using one of the A-frame ones, which provide surprisingly little space for the birds, esp. indoor space. A box- or rectangular-shaped one is not much harder to build and will give you a much more functional multi-season product.
The ideal solution though is probably to have a tractor for summer use and a coop for winter. BTW this also gives you somewhere to expand easily into, at least til wintertime
, in the event that you find yourself wanting to acquire more chickens, which seems to happen to most of us
Good luck,
Pat