Sometimes heat can cause them as heat is stressful. It's not from a lack of calcium, shells get brittle usually when it's that.
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so it's not due to the heat or lack of calcium ?
When my girls first started laying, they all laid at least a couple soft-shelled eggs to start...then once their system regulated all was fine. Also, to answer your earlier question, chickens pull calcium from their bones for eggshells so that's why it's important to have oyster shell and/or layer feed available. Congratulations on your first eggs!
check the bottom of the feet. Yellow would be Jersey giant, and pink would be Australorp. My guess would be the darker eggs or the one shade lighter.