Hi Azygous-- I believe Calcium Lactate may be more bioavailable than Calcium Citrate. I am investigating this because I have a duck and a hen I have been giving calcium citrate to and the shells remain thin. I am going to try other forms of calcium.
I had a eggbound Indian Runner with a soft-shelled egg. She has been laying soft-shelled eggs no matter what I offer. It finally came out after a calcium pill and 2 doses of Calcium Gluconate.
I have a duck with a soft shell hanging out of her vent. I wonder how they can push it out if it is collapsed. I lost a duck to the exact same problem a few months ago and when I pulled out the crumpled compressed soft shell, I caused prolapse and the duck died after a couple days.