My birds are good to at least -15 without heat, your adult birds should be fine in the 20s. How old are the "little" ones? If they're still chicks, they'll need heat. The thing about heating them as adults in the winter is if they're acclimated to say 45 degree weather with the heat lamps, and the power goes out and it drops to say 15, they probably won't handle it too well. I've also found that the birds actually do better without the heat bulb or heated water...that's not saying I don't bring them out warm water to keep it from freezing faster, but they don't seem to do as good on water that's constantly warm.