Shetland lover
Free Ranging
I've only had brittle (or even shell-less) eggs when my girls have gotten towards the end of their laying life - and then only with my hybrid, ex-battery girls. I increased their calcium intake by adding a powder or liquid calcium supplement to their food or water and increased dark, leafy greens. I also fed back ground eggshells and a can of salmon or sardines with bones. Made sure i never fed spinach or other plants high in oxalates (eg sorrel) to the affected ladies as the oxalic acid strips calcium. It helped in the short term but obviously being at the end of their laying careers, it won't help for ever. A younger bird experiencing a deficiency may respond better long term.One of my chickens started molting last night, lots of feathers scattered around and she's looking scraggly. I gave them all some cooked pork today for extra protein.
I hope their egg quality improves soon. One or more of them keeps laying thin shell brittle eggs, which get eaten in the nest box after they break. At least they're not going to waste. They don't bother with the solid shell eggs at all, so it's ok with me that they eat the bad ones.
Mine have never eaten eggs in the nest box, thin-shelled or otherwise, but any dropped on the floor are fair game. I'm also comfortable with that. I'd rather the girls got the benefit than the egg go to waste.