When you hear someone talk about a pest infestation, your first thought might be roaches or bedbugs, but not birds. Pest birds breed in closed-off spaces such as the attic of your house. They can damage your house and spread diseases leading to infestation by other creatures such as mites and ants.
The dirt and grime from the atmosphere can really get into your home. You can ignore it, but it will still be there. It makes the air in your house more difficult to breathe, and you might suffer from allergies or other respiratory irritation.