The mess… you can get staining on the wall if you mount it on the side of the house. I mean quantity really depends on species and how many take up residence, but you certainly wouldn’t want to mount it right over a seating area or your lettuce patch.

(Although the droppings make great compost ammendments.)

Generally, I would recommend mounting on a pole, 15-20 feet above the ground with several feet below clear of shrubs etc so they can safely drop into flight.