They’ll tell you when they’re cold, but personally, I think it’s cruel to have them withstand subzero temperature environments. Sure, they might survive with cover, but imagine how miserable you would be in the same temperature extremes (even with a jacket)?
If it’s 30F or warmer in their shelter/environment they should be fine, but if you don’t have a heated area for them, it’s better to just bring them in. Insulated huts don’t count as a “heated” area - in my experience once it drops to below 15F the insulated huts don’t do too much.
Maybe it’s because I witnessed first hand what happens to feral cat colonies exposed to subzero temperatures… they die of exposure. There’s nothing like shoveling out snow and finding a frozen-over body of a poor cat that died during the cold snap.
Our little kitty, who is very, very hardy, started showing signs she was uncomfortable in around 10F degrees. She has an insulated kitty hut in our non-heated feed shed that I made for her, lined in a warm fleece blanket, and she would not come out brightly. She’s normally a very cheery and friendly kitty, so for her to refuse to leave the hut tells you she’s cold! She’s in the house now.
We’ve had wild temperatures from 30F to -5F the last few weeks. She’s fine in the 30F weather but the night temperatures swing wildly and we’ve been scoured by extreme wind as of late.