Concrete flooring, NEED ADVICE PLEASE HELP

[Buck Oakes]

Tried sand, they ate it and the rain also made it go away

 

[TLWR]

The floor of my duck house is trex decking. They also have a concrete mixing pan full of pine straw in there that they take turns snoozing/pooping in.
I hose out the house daily and empty/hose out the bin of pine straw when they've gone crazy pooping in it - usually can go a few weeks at a time.

If you do concrete, keep an eye on their feet and give them an alternative that has something with a bit more give to it.