I use zip ties for chicks and check them frequently changing them just before they start to get snug.
I found lots of colors in a single pack at Harbor Freight. When they grow up they get numbered legbands.
I used net produce bags and quart fruit baskets upside down to segregate them in the...
I'm guessing thick shells may be just too hard for the chick to break free from.
I also think they can contribute to small air cells because there is less moisture loss. If that's true, then I imagine less oxygen is getting in.
I can't candle Penedesenca eggs either.
If I float them to find the air in the round ones I add a teaspoon of bleach to the warm water.
I read on a French Marans club site to set the temp 1.5 degrees higher.
I set mine 1 degree higher for Penes and it worked out pretty good.
Some Marans breeders...