One of my hens didn't return to the coop tonight. I'm hoping that she's just roosting somewhere outside and hasn't been killed. I've looked for her and called her but no luck. I've had chickens hide at night before only to reappear in the morning. But tonight it's supposed to be around 18 degrees. She's supposed to be cold hardy (I believe she's an ISA brown) but I'm not sure how hardy that is. Do you think she'll be ok outside overnight or will she freeze to death?