The biology and chemistry of pigeon poo
Birds differ from mammals in numerous ways and one of these is that birds do not have a bladder, if ever you've held a bird in your hand, you'll be amazed at how light they are. It makes sense when expending considerable amounts of energy to take flight and battle head winds that weight is going to be a key issue, and so birds mix their liquid waste from their kidneys and the solid waste from their intestines in an organ called the cloaca.
If you watch a bird that has perched for any length of time, you'll notice that they often defecate just before they take flight; they're expelling all their waste in a liquid soup and this soup takes the form of concentrated acid. The pH levels of pigeon poo are somewhere between 3 to 4.5; for comparison lemons are around 3, acids will be their nature try to neutralise themselves by pulling hydrogen ions out of their surroundings. The bird poo that lands on your building is reacting with the material it sits on slowly breaking the surface down.
The problem for all your different surfaces whether its paint, wood or any porous surfaces is that the acid will enter through microscopic flaws and entrances slowly working deeper into the material and eating away as the insides. Rain, freezing weather, sunlight and wind all do the rest and weather the material breaking the surface further and further, it takes years but we'll often see badly damaged surfaces that have been buried under pigeon guano which could have been avoided.