The Black What?

I've been changing the shavings in my aviary, and every day there are piles of new shavings around the bag. I've found this puzzling. Why would chickens be attracted to a bag of new shavings? No food there.

Today I turned around while changing waters, and was startled to see a black mop-looking thing lying in the bag. What the heck?

It then started to move. As it moved it morphed into a hen.

She stood, pecked through the shavings to loosen up another level, and resumed bathing.

I guess if your choices are snow, mud or shavings, shavings seem like a delightful bath material. I'll have to bring in a box of dry dirt or sand for them. There isn't any of that outside these days.