They stick their beaks into the mud and gurgle it about and then proceed to clean their beaks by dunking it into the water, which causes it become instantly dirty and ready to change. It's so annoying!
Link to video: https://drive.google.com/file/d/1CBVE2MIzDIoAs172VlC3ddPxxzG0aSLZ/view?usp=drivesdk