Luv's right... and I'm feeling the need to clarify my stance on this...
It all depends upon the number of birds you have, the breeds you have, the age they are and the living conditions you have supplied for your birds.
I have many birds, and yes, it gets extremely cold here and they're fine. But I have chosen cold-hardy
standard breeds, most are full grown, and they have a fully insulated coop. I'm watching the birds carefully, conducting my own experiment if you will, to see what temps/humidity the birds start showing signs of discomfort. Currently it is a balmy 21°F and 91% humidity.
For people who live in relatively tropical areas that are seeing the rare event of temperatures in the 30s, 20s, and teens, just watch your birds and if they start acting uncomfortable for the conditions they're in, then yes, give them heat. I'm guessing that they'll be ok for the most part, but some of the smaller breeds, younger birds, or very old birds, are going to be stressed by the temps. If you give them a heat lamp anyway, they're going to huddle under it. Wouldn't you? I'm not saying that you
shouldn't give heat... just that for adult birds that have been allowed to acclimate with the changing seasons that have a draft-free dry coop and a few other adult birds to cuddle with you shouldn't
have to apply heat. When people say "Oh, it got down to 35°F last night, I'm going to put heat on my birds!" I have to ask "why?" They have down coats. Unless they're wet or drafty, they
should be fine, unless they're very young, very old, very small, sick, molting or all by themselves... yada yada.
OK. I'm done with this subject.
My fingers hurt...