I use a #2 pencil,, but I think under a hen, the pencil might get rubbed off..
I am against using magic marker to write on eggs,, but something like just a dot might not do any harm.. ? or maybe crayon would be better ?
I get eggs written on all the time. Never had an issue with it. Hasnt reduced my hatch rates any. chicks seem fine. Pencil does wear off or get so wiped off it isn't viewable. Crayon also rubs off under a hen.