they say if you use any kind of heat in the coop you could cause the chickens harm when they leave the coop, as the shock to their system would plummet their temperature too quickly.
I think we have to consider this rationally.
IF it's super cold out, AND you heat a lot, THEN maybe it would be tough on them...but their feathers have the same mechanism as other birds and they can "fluff" them out to trap warm air against their bodies and stay warm (as long as they are dry). This is, I believe, the same reason that a windy day means they can tolerate less cold, because it blows that trapped warm air away from their skin (just like why windchill doesn't lower the actual temperature, but does mean you get to that "real temp" quicker - and why warm clothes work...they aren't warm clothes - you are a warm body, and they work to trap the heat YOU produce against your skin, instead of having it dissipate to the environment.)
It all depends, I think, on what the actual temperatures ARE. You're just not going to freeze to death at 17C/63F, even if you feel a little chilly if you're used to something warmer.
Consuming more energy/food is necessary as the temperatures drop because that's what your body uses to produce heat energy. To a point, that will be enough to allow your body to stay warm. Birds have the advantage of having mobile "clothing" that they can puff up or sleek down to trap more or less body heat.
I just don't think if they have free access in and out of a heated space, they are at THAT big of a risk of being harmed by the temperature difference, although you can probably conserve energy and money by hitting the right point of balance, and a lot of people dislike using heat lamps because of their concern about fires. Chickens will decide where they are comfortable when they have a choice, and people are observing their hens going in to warm up and coming back out as they please.
I agree with other comments that being dry is a big deal, because wet feathers (and clothes) aren't as insulative any more. Wet skin also cools faster, which is where frozen combs and wattles become a problem.
Not providing heat to my chickens would be abusive, imo, because it is VERY cold here and they are VERY small. Not providing heat in warmer climates (including those that dip below freezing), or with more chickens of larger size is really not such a big deal. We kept larger chickens where it regularly dipped to or below -40 (F or C take your pick) and other than a few comb tips that froze, they were fine without heat. They would have been fine with heat too, I'm sure.
My difficulty is knowing what these very different birds I now have can handle, or I wouldn't be following this thread. You have to know what temperatures you're talking about, both inside or out, and I think blanket statements aren't as helpful as knowing the reasons and the limits. I have two heat lamps, one small, one large, and I will use the one that seems appropriate based on what I am observing, and the ambient temps.
I feel a little worried that this post will be misinterpreted, but having struggled with what to do with my own hens only because they are so tiny, and having watched them over this last very cold week (-26C/whatever that is F) - I'd rather know what signs to watch for that they are warm enough, vs being told not to provide any heat without considering what actual temperatures I'm dealing with, and the specifics of my little flock.
