soooo interesting.
Someone somewhere referred to chickens as 'cavity nesters'. So in the wild (which isn't where my chickens live ;O] ), a chicken would find a suitable cavity... a nest resembling that shoud be attractive.
They want it to seem "safe" too,
Don't forget the trick of putting golf balls or fake eggs in too, that way they think some other chicken already decided on a good place to lay so they will copy that chicken.