Rather than just marking them with a number or date try drawing a line the whole way around the egg. It should also help prevent marks from being rubbed off. You can still number them for tracking purposes, but the line will show no matter how the egg is laying in the nest.I have a d'uccle hen sitting on 6 eggs for a week. tonight I went to candle and found an egg that wasn't marked. I thought good. Unfortunately, I found one in the house that was marked. I hope I didn't kill it since it would have been in an 80 deg. A/C house for about 4 hours and rinsed off. I guess it doesn't do much good to mark the eggs if you don't actually look at the entire egg for the mark. LOL