Thanks for the replies. My friend kindly picked up a huge bale of straw for me - a "coop warming" gift, if you will. So at least for the moment, I will be going that route.
The crop (if that's what it is) is very soft and pliable. I will watch it. I've been giving them a lot of yummies besides their chick food; wet almond meal left over from making almond milk, loads of carrot, pear, spinach,kale, apple pulp from the local organic juice shop, juicy cantaloupe rinds... My family is BIG on produce.
Maybe I'm overfeeding?
We're big on produce too.
