The hens will not sit on their nests until all of their eggs are laid. That's why extremely cold weather could hurt some early layers. The eggs are pretty tough though. A little snow doesn't hurt them. Anything laid now is fine. The biggest problem is nest raiders like coon or skunks. Next on the list is probably mushroom hunters. DNR research has shown that once a hen starts to incubate the eggs, sitting 24 hrs. a day, even being flushed off it once may cause her to abandon it and start over. The longer she has been siting on it the more likely she will return to it but the bottom line is that they shouldn't be disturbed.