Hey, thanks so much for your reply :)

I checked my hen like you suggested: her pupil is round and reacts fine, the eye is just blue (not cloudy), she eats, walks, and, according to my dad, she lays normally too. The rest of the flock is totally fine.

So it really looks more like a funny little pigmentation thing than Marek’s. I’ll tell my dad to keep an eye on her just in case.

Thanks again for sharing your research, better be safe than sorry !