Depending on how many hens and what color eggs, sometimes it works to divide them into several pens. If each pen has one hen that lays white, one that lays brown, and one that lays blue, it is easy to tell who laid which egg.
Of course that does not work when someone has a flock with one...
That is a good idea, to see if the problem is the same across all hens or if it is only specific ones.
Although the food coloring idea doesn't always work as well as it sounds. I've tried it and gotten colored eggs, but I've also tried it and gotten non-colored eggs, even though I thought I did...
I'm wondering if there could be some kind of nutritional deficiency, because some of those also cause hatching issues before they cause obvious symptoms in the adult chickens.
Unfortunately I don't know what deficiency it might be, but I have sometimes found big charts or tables of all the...