Can I stop birds from building nests on my house?

Birds find all kinds of places on houses to build their nests. The porch light fixture is a classic example. It's elevated, stable and flat on top--perfect for a nest. The best prevention for this is to change the flatness on top. A forty-five degree slope will make it an unbuildable site for birds. Another way to stop nest-building is to attach a vertical mesh barrier to the spot.