The next time you are tempted to complain about birds using your freshly-washed car as a toilet, consider the important role bird droppings have played in the history of the United States.
Prior to the development of artificial fertilizer, bird droppings (more politely referred to as guano) were in high demand as natural fertilizer. One of the best places to find guano in large quantities is in small, uninhabited tropical islands. In some islands, guano had built up to over 150 feet in depth. (For an excellent discussion of the 19th-century guano industry, see here.)
Since the acquisition of guano was so important to the economy of the United States, Congress passed the Guano Islands Act (48 U.S.C. § 1411-19) in 1856. This law declares that if any US citizen discovers an uninhabited island that is not claimed by any nation, and that island contains a sufficiently-interesting amount of guano, that person may claim the island on behalf of the United States. If the President then considers the newly-acquired real estate worth keeping, the person who discovered the island may retain the exclusive right to the guano, and the President would be authorized to deploy the military might of the United States to defend the island.
In short, if you find an island covered in bird poop that no one else claims, you just added to the territory of the United States of America, and your reward is that you get to keep the bird poop. More than 100 islands around the world were claimed by the United States through this law, including Midway Island.
This law remains on the books, and it was invoked as recently as 1997 when William A. Warren attempted to claim Navassa Island (located near Haiti). The U.S. District Court of Appeals for the District of Columbia Circuit rejected Mr. Warren’s claim to the island for several reasons, not the least of which was that the U.S. already claimed an interest (contested by Haiti) in Navassa Island and has done so since 1857. Be that as it may, you still have the opportunity to expand the borders of the United States if you happen to stumble across just the right piece of guano-encrusted real estate.