Some time ago I posted reporting a "rare" event: three shell less eggs in one day. Several member answered. Thank you all.
Most likely causes:
1) Lack of Ca: The eggs, when shelled, have an extraordinary strong shell.
2) Inappropriate nutrition: I am giving them pellets for laying hens (May I say the brand in this forum?) and oysters shell. Besides lots of greens from my garden (100% organic): spinach, beets, collards, kale... you name it.
3) Inappropriate housing: The coop is 8'X3'X8' half open in one side. The coop is securely closed after they perched and open very early in the morning. The chickens enjoy an area of more than 600 square feet.

With this flock, all of them ameraucanas, the frequency of shell less is more than once a week.
Now, I have had chickens before and have seen shell less eggs and dwarf eggs from time to time. But I have never seen this kind of siamese phenomenon nor this high frequency of shell less eggs. Have any of you? Do ameraucanas have this kind of problem?