Completely 100% disagree on straw being eggcellent at moisture absorbtion.
40 yrs of having horses has told me one thing - straw sucks at absorbing and holding moister and at reducing odour. This is why I under-bed my straw bedding with softwood shavings in the horse stalls.
For bedding - something that an animal will lay on and poop on:
Best material is mixed size softwood shavings. Absolutely not toxic - hardwood shavings yes have high levels of toxic chemicals. Do not ever use hardwood shavings with animals.
Clay and sand are very good at holding moisture. To understand which is best one needs to understand the nature or soil. Clay has very fine particles which hold moisture well (this is what kitty litter is made from), sand is more course and will contain moisture but not absorb like clay. Sand drains well, but in the winter freezes like cement when it gets wet.
One other thing, once the chooks start pooping in the sand if you don’t pick poops daily the chooks will scratch around and break up the poops causing them the mix with the sand creating an organic composition that hold water and odour. Yep the same will happen with shavings, but the odour is held better in shavings, and it won’t freeze in the winter.
As far as bedding goes, soils are more used as flooring materials, though I have seen sand used in horse stalls in Egypt.
If one is wanting to have a flooring that is easily picked through for poops and such, then use course sand.
But personally I like softwood shavings - they are non toxic, highly absorbing and eggcellent at odour control.
One thing I should also point out is that I personally pick the hen house daily of all poops, that includes the roosting ledges and the floors. Takes all of 5 minutes when done daily, this goes a long way towards controlling odour and moisture.
EDIT:
- sand and other soils are very heavy especially when wet. So if you are old and decrepit like me use something that weighs less

What experience do the rest of you have?