During the mid-19th century, rapid industrialization and intensive farming swept across Europe and North America, severely depleting agricultural soils of vital nitrogen and phosphorus. The agricultural savior turned out to be guano—accumulated bird droppings—which dried in massive, nutrient-rich deposits on arid islands off the South American coast. Recognizing Peru's lucrative monopoly on global guano sales, the U.
S. Congress took unprecedented legislative action by passing the Guano Islands Act of 1856. The law stated that whenever any U.
S. citizen discovered a deposit of guano on an unclaimed, uninhabited island, rock, or key, that island could be considered, at the discretion of the President, as appertaining to the United States. Furthermore, the act granted American citizens exclusive mining rights and allowed the U.
S. military to protect the claims. This legislation marked the first actual overseas imperial expansion in American history, preceding the annexation of Hawaii or Puerto Rico.
Over 100 islands, atolls, and reefs were claimed under the act during the late 19th and early 20th centuries. Today, several U. S.
territories, including Midway Atoll, Baker Island, Howland Island, and Jarvis Island, remain under American jurisdiction purely because 19th-century sailors were hunting for bird manure to fertilize domestic crops.