Pest birds can cause a build up of debris on roof which can lead to considerable damage. Nest materials, feathers, and fouling can accumulate and block gutters and downcomers leading to leaks and floods.
Bird fouling, known as guano, is also corrosive because of its acid content. This can cause damage to roofs and buildings. If plants grow on bird droppings because of dropped seeds for example, this can cause even more problems.