Duck Laid A Shell-less Egg

[Pollo Blanco]

I've had a stressed duck lay eggs with only a membrane and no shell twice.

I've also had a few thin shells

Most of my eleven hens lay normal eggs, so I am not worried about a calcium problem.

Most of the grit that they consume is crushed limestone

 

[Isaac 0]

Calcium citrate, carbonate (found in tums), gluconate; all will work to a degree to help correct a calcium deficit. Unless one extrapolates literature of calcium bioavailability done from humans, it's hard to say which one will work best without it being formed on an anecdotal basis. In humans, citrate is superior in absorption to carbonate.
It is discussed with poultry here:
https://www.sciencedirect.com/science/article/pii/S1056617120301094
but, not to the extent one could form a proper conclusion of which might be best.
Regardless, all have been mentioned as suitable supplements in avian medicine, so try whatever, and see which works best.
I have personally used gluconate, and dosed birds 0.2ml per pound, orally, or subcutaneously. Besides calcium, there are other factors such as stress and young age that can create the onset of soft-shelled eggs.

http://avianmedicine.net/wp-content/uploads/2013/03/18.pdf

 

[my gang eats grass]

I wouldn't be worried if she only lays one, but if she keeps laying them consistently give her a calcium supplement.