There is no repellent that will work on birds, as they do not have an acute sense of smell, and things like moth balls or amonia will not repel them and keep them from building their nests.
"The U.S. Fish and Wildlife Service wants to remind home and business owners that the time to remove a potentially messy or hazardous bird nest is while it is being built. If you notice birds beginning to build a nest on your property in an inappropriate place, destroy it immediately to keep yourself from ending up with a nest full of baby birds in an unwanted location.
Because migratory birds are protected under the federal Migratory Bird Treaty Act, it is illegal to harm, kill or possess them without a permit. Their eggs are also protected. Destruction of an active nest—one containing eggs or birds—is against the law. However, migratory bird nests may be destroyed without a permit as long as the nest is inactive (there are no eggs or birds in it). Destroying nests before they are completed will save you a mess and could save you from violating the law.
State laws may be more restrictive, so check with your local Department of Natural Resources to be sure destruction of the nest does not violate state regulations.
If a bird nest contains eggs or young, you will need to look for ways to live with it until the young are old enough to leave the nest on their own. Landowners are expected to be vigilant before the nest becomes a problem, and removal after the eggs are laid requires a permit which is both costly and usually difficult to justify. Recognizing potential problems before they become real problems is the best way to deal with inappropriately placed bird nests." http://www.southeasternoutdoors.com/wildlife/birds/articles/prevent-problem-bird-nests.html