Pigeons often build their nests near drains and inside of gutters. One pigeon can produce up to 25 lbs. of poop a year and will nest inside of their own feces. Pigeon droppings are acidic enough to eat through concrete and rust steel. Without regular cleaning the pigeon droppings will accumulate over time clogging drains and adding weight to roofs, canopies, and other structures. When water is added through precipitation the problem can cause roofs and gas station canopies to collapse. Read about one instance in northern California here.
In severe cases where a building has been overrun by a flock of pigeons we always recommend removing pigeons with a trapping program first. After we get rid of pigeons that are roosting or nesting we will pressure wash and disinfect the areas making them safe for its occupants. The last step is to install steel screening, pigeon netting, or stainless steel spikes to prevent future pigeon nesting or roosting on the structure.