I am wondering the same thing. I bought a box of bird grit and it is a lot smaller than regular sand. Grit has fine egg shells for calcium, I think, which we can add to sand if it will work. I hope regular sand is fine enough grit for them because it's cheap. I think it sounds like a better idea to get a bag of white play sand from a box store; it's probably cleaner. Heck, or maybe its treated with nasty chemicals! That wouldn't be good. I'm thinking river drag would have nasties in it.
I am wondering just how resilient chickens and chicks really are. I have my first 26, 4 day old chicks. If take them to the garden and on a day when it is 85-90 degrees, how does the sun treat them? Do they overheat? I have noticed that they will be walking along on the warm ground and just fall over asleep. It's funny.
I am also curious about what I have been reading about using clay to dust them with. I have some and I would love to hear from someone's personal experience with using kaolin clay or bentonite.