Yeah, even if it truly was spring, it was a idiotic move to not peek outside, because spring won't be "full of mister munch munch, mister flowers, mister bugs, etc." until, at least 1 week or 2 after it started, so they would starve to death without any food for the first days of spring, since all the snow is still melting and the flowers still need to bloom, aside of other animals waking up from hinbernation.
I'm guessing yukkuris usually time their emerging from the nest to coincide with when there's food outside and most the snow is melted, which is when things would be warmer.
Jacking the temperatures inside the nest up real quick and high would pretty much short out their thought process so they wouldn't think to check first.