Recently, I keep finding the bird droppings on the deck. Are there any ways to keep bird from leaving their droppings on the deck? Besides, are there any good chemicals that can clean the bird droppings without affecting the deck paint? Thanks.
Give the birds somewhere better to roost and poop. Put up a clothesline or other string-type thing over a patch of ground that could benefit from fertilization. The birds will roost on the string and poop on the ground, fertilizing it. Now you have a clean deck and your soil is receiving a regular infusion of minerals, particularly phosphorous, which is otherwise hard to add to soils.
I have great luck with a motion activated sprinkler Something like this keeps the geese out of my pool, It should work for your deck also. Cleaning the stains may take a strong deck cleaner but not knowing the type of paint I would hose it off and try a little dish soap with a scrub brush to clean it off.
On my deck, bird poop tends to happen where the good tree branches for hanging out in are, particularly near the bird feeder. So I'd start by looking up and asking "What's up there (or nearby) to make pooping right there popular?"
If I minded more, I'd start by moving the bird feeder. If I still was being bothered, it might be time to remove a tree (we removed a formerly popular one for more practical reasons having to do with it wanting to crush the house, and that area of the deck has much less poop now.) In practice I can cede the birds a certain portion of the deck, so I don't get overly concerned about it.
Di-Hydrogen Mon-Oxide is a perfectly good chemical for removing bird poop from the deck, and it's "safe as water." Apply liberally, brush if deposits have dried and are stubborn, then apply more.