An old trick, I’ve been told by several women in my life, to whiten sheets and linen, etc., is to throw them out over bushes and leave them there for some time. The thought is that the oxygen being generated by plants (maybe even ozone or O3?) contributes to the process. Maybe the pseudo-shaman was right! (?)
The white is from the bird shit.
Sunlight bleaches, but I don’t know how long you’d have to leave something out for it to be noticable.