Dirty Butts! What should I do?

I'd either wash them, or trim the feathers off around their back ends tp make the birds more comfortable. You might want to take a close look at them to be sure they don't have Vent Gleet or something else going on. https://www.backyardchickens.com/a/vent-gleet-aka-nasty-chicken-butt They are acting fine otherwise, are they laying?

 

A worm infestation can also cause their backsides to be dirty...have you recently de-wormed your flock ? If not it may be prudent to treat them for any worm problems...