Pest birds are capable of spreading diseases to humans and livestock. Pigeon faeces often contain the fungal disease histoplasmosis which is especially dangerous to people with compromised immune systems. The droppings of most pest birds create aesthetic problems. They can also create drainage issues by nesting in the small crevices, drains, and gutters of buildings. Pest birds serve as hosts for parasites like fleas, mites, and ticks. Modifying their environment can help property owners in ridding buildings of bothersome birds. Netting or spike strips can be positioned to deter or prevent birds from landing and nesting. Noise devices that mimic the sounds that natural predators and visual scare tactics are effective when paired with other prevention and exclusion techniques.
http://www.delta-optimist.com/opinion/blogs/deltabc-1.983313/blog-pesky-birds-1.2027469