I'm in the South Valley, too. Far South Valley where it gets much colder than in town. Coops don't need to be heated, but they do need protection from wind, and straw is nice, so if your tractor coop doesn't have an enclosed area for them, you may want to fashion some siding.
I don't know very much about raising chickens, but my brother (who lives in Seattle) has a bunch. He found last year that when he did put a heating lamp in the coop, egg production went way up (it had been declining as things got colder). So, maybe not necessary, but it may help boost production during the colder months, even in varieties that are cold hardy. Another consideration. Have others found this to be the case, too?
I Karla. I have the birds from the other post, so I wanted to respond directly to you on this post. Someone else passed on to me that birds were ok down to zero degrees, as long as they are protected from the wind and you feed and water regularly.
I can also tell you, remember how bad that last winter was here? We made sure the ladies were shut up at night and we put a low voltage seeding mat in the coop to be sure. We didn't have any problems last winter, and this winter we did away with the seeding mat, no problems.
Chickens will survive a lot of cold but if laying eggs is wanted more....a light bulb in a closed coop may be enough but if it is too poorly insulated it may need more heat. Chickens in straw is more comfortable for them as well. What I hope everyone avoids is what I have seen in some poorly insulated coops where the chickens get discolored cones and body parts for having been frostbitten as I think that is inhumane. It is dangerous to put in a heat lamp with any kind of animal where the heat is intense.....and they repeatedly use it and then go outside as they will make themselves sick by engaging in this cycle of behavior. The heat lamp is okay if high on a ceiling however but not at the face level of the animal.
My chickens are in a coop with a door, inside a shed converted to a chicken run with only chicken wire on two sides. They seem to be just fine. I had a light on at night for them just to lengthen the light hours, but a friend of mine doesn't use any light and still gets a good supply of eggs. So I've turned my light off for a week now and they're stlil laying at the same rate.