I would like to know what type of pen you all use to mark/write on eggs, please.

Sharpies! They don't leak through the egg and they don't crack the egg. Mark the air cell.

Thank you Baruch! I never heard of Sharpies before.
I read that they are permanent markers: will any brand be ok or the Sharpies are the only ones safe to use?

Any nontoxic felt-tip marker will work. Sharpies come in a variety of tip sizes. There are lots of different colors too but this picture only shows black:


Cockatiel eggs are small and hard to write on, so markers with a thin tip are good!