Well, I found another bad egg today, which means another one of my quail hens has been having the same problem. Unfortunately, I am not sure which one it is. I am guessing it is one of my two older hens (both 3 years old), and indeed one of them has been acting a bit lethargic for a while, but I cannot be sure until I see her lay a bad egg or the other hen lay a normal one. That, of course, is unlikely to happen. There aren't any exterior physical symptoms I can look for on my quail, are there?
Here are a couple pictures of the bad egg. The shell was weak, hence the hole, and as for the coloration, it obviously indicates a problem. I cracked it open in the compost bin, and it did have a normal yolk.
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