I don't speak from personal experience but I've read a few Sour Crop threads in which some of the OP's think their chickens (probably craving grass or greens) got too long of pieces of straw and hay and developed sour crop. I think it's a bird specific thing too...because really hay and straw have been used for ages for chickens. But if you want to be really pro active and protective of any issues...I guess wood chips or shavings would be the way to go to avoid those problems in the run. Some Runs are pretty big and one of those posters suggested contacting a tree trimmer whom I think are always looking for a place to get rid of the branches and leaves they chip up. Also I think another poster said for winter her birds did not care for the cold sand in her coop. I guess wood shavings do sound cozier. Today I mowed my lawn and I brought all the clippings into the run and I'm hoping my birds will just munch on those this winter when they're wanting something "green".