Since it rarely gets that cold here, I can't tell you. But maybe one of the others can. I know when I let them out usually the eggs are pretty warm.I am self employed. I work on my ranch so checking the trap nest would not be a problem.
I have found that the fertility rate drops in my eggs when the temp drops below 38.
three weeks ago the temp was 40 + and the eggs i collected had a 74% fertility.
the batch of eggs i started last week was under 52 % the temp was 34.
that is why i was wondering if the hen would keep the eggs warm while she was in the tarp.