If subjected to rain, the majority of the pigeon poo will wash off. But inside attics, ducts, or some other place within a building, under cover of roof or under ledges, the poo will stay, and it has to be cleaned.
Inside, however, that is not feasible. You will want to vacuum it out, with a really good filter vacuum, and suitable protection including a HEPA filter mask. Absolutely don't inhale the spores or dust from pigeon feces! Spray down, or even better, fog the whole attic or inside space using a distinctive microbial and enzyme based cleaner which will clear out all of the leftover feces, pigeon urine, and bacteria, in addition to mold, spores, dangerous pathogens and microbes. Goods like Bac-Azap or BioShield are great for cleaning.
Elimination of these wastes is essential to prevent certain ailments like histoplasmosis, psittacosis and the incidence of another virus called Cryptococcus which may cause fungal meningitis in individuals. To elaborate, histoplasmosis is also a fungal disease that's sometimes also called "cave infection","darling disease", and lots of others that's caused by the fungus histoplasma capsulatum. It mostly appears as somewhat as a respiratory disease which affects the lungs. Last off, cryptococcosis is the disease caused by the virus Cryptococcus which may also be found in pigeon droppings.
To be able to avoid acquiring the aforementioned diseases in addition to the numerous side effects it may cause in your own lives, it's much better to clean up all of the bird droppings you may see as much as possible not just for hygiene purposes but also to eliminate the chance of diseases caused by inhaling the bacteria and viruses found in their wastes.
Of course you're expected to exercise a large deal of caution when dealing with pigeon guano. It's mostly recommended that you protect yourself by covering your face to be able to minimize inhaling the odor and some microorganisms found therein. You need to also wear plastic gloves so as not to contaminate yourself with some of the pigeon droppings and besides this, you can wear long clothing to prevent getting the wastes into any part of your body but if you're really careful you wouldn't permit yourself to be splashed with their droppings unless they're still roosting exactly over the spot where you're standing. It isn't really the dirt or how you've got it on yourself that's dangerous but if you inhale the substances which may cause the diseases.
You may try to scatter them with water so as to wet them and prevent it being carried by the wind to other areas and most importantly to the passersby who could inhale it. You should also take care to not actually wash it away because they may be carried into another area where bacteria can still grow and may be more dangerous. If need be, you might have a bucket of soap and a brush to scrub them off the floor of your residence or some other areas of the building.
If left unattended particularly in roofs, you may find yourself having problems with clogged gutters or any other tubes or pipes. That's the reason it is also vital to be certain your roof is free of those pigeon droppings. Also it's strongly recommended that you assign a particular dustpan solely for the purpose of sweeping pigeon guano rather than for anything else merely to exercise caution of potentially spreading the germs which may be found in their wastes. You might also attempt to set them in a plastic container and seal it to prevent the odor from escaping.