I think I finally figured out the difference between my Silkies and d'Uccles: My Silkies lay a light cream colored egg with a faint shiny sheen on it. The d'Uccles' eggs are a shade darker with a matte finish.
I got 10 eggs today! That's a personal record for me!
I have 6 EEs and it seems that I've had about 30 variations of blue/green eggs varying in size and shape. I've had some double-pointy-ended eggs and some as round as a golf ball and few double-yolkers.
I thought about trap nests, but I could see me forgetting to let the girls out and them...
Anne, you're too much!
Okay, next I suggest that you take each egg, blow it out and mount it in the kitchen with the hens' name under it. Then when you get a mystery egg, you'll have a whole line of eggs to compare it to! Of course, you may have to have multiple entries for each chicken...
I suggest you take some time off of your job and set up a blind (like a deer blind) with a good view of the nesting boxes (some binoculars might help) and watch them over a period of several days to see who goes in what nest box. After they leave, you immediately run over and snatch the egg...