Feral birds can cause a number of problems in and around a building. Pigeons and seagulls can force an entry into a building finding an access point under roof tiles displacing roof coverings allowing water into the building with subsequent decay inevitable. They can also block guttering drainage systems with their faeces and other detritus.
During nesting pigeons, starlings, etc will block gutters and downpipes causing complete blockages in a short time leading to serious drainage problems particularly damaging in unoccupied buildings where the problem is less likely to be noticed early on. Pigeon faeces and detritus are both an aesthetic and public health issue, when pigeons enter a building droppings quickly deface the interior and exterior causing expensive and time consuming cleaning and removal of faeces.