I have observed the differences in when the babies open their eyes and have found that in general larger babies open thier eyes earlier while runts (or smallest in the litter as I dont generally keep any proper runts) can often be several days later in opening their eyes. Its a general rule though and some cases just dont follow it, recently one of the smaller babies in my litter of blues opened his eyes at 10 days old the others didnt until day 13, and they all opened theirs on the same day.
I use hay for my mice to nest in, they live in the garage so they need the extra warmth that hay gives them, shredded papr works too but I find it gets damp quickly from the condensation. Anyway I think using hay can lead to some eye issues, either the particles of dust getting in there or simply being poked in the eye by a sharp bit. But as I said mine all clear up in a few days, I dont think its uncommon.
