The Guano Islands Act

Guano. The dried, hardened white substance left behind by seabirds, seals and bats when they defecate. Quite literally, a bunch of bird crap. Not exactly glamorous or pleasant. And yet, it’s the foundation of one of the world’s great empires.

Prior to the invention of artificial fertiliser at the turn of the 20th century, manure was the world’s major additive for keeping agricultural soils full of the nutrients required for intensive crop growing. With its high levels of nitrogen and phosphorus, guano is especially potent. When this fact was discovered in 1804 by Alexander von Humboldt, guano suddenly became a very valuable commodity, and by the 1840s was a major source of fertiliser. Added to its agricultural use was its high sodium nitrate level, valuable for use in explosives. For an expanding country such as the United States, which required both fertiliser and explosives to fuel its economic and military engines, obtaining and controlling sources of guano was vital to the still-young country’s success. As the 1840s became the 1850s, more and more small, remote islands in the central Pacific Ocean (and to a lesser extent in the Caribbean Sea) were found to contain sizeable deposits of guano, often dozens of metres deep. These islands shared the two things necessary for producing mass deposits of guano: large seabird populations, and very little rain (thus preventing precipitation from washing the dung away). These islands, for the most, also shared another thing in common: they were uninhabited locales that were not claimed by any other country (or at the very least, were claimed but unmanned).

The guano hotbeds to this point were Chile and Peru, thanks to the presence of the Humboldt Current (you can imagine who that was named for), which produced a dry coastal climate perfect for generating guano.Entrepreneurs began looking to break the Peruvian hold on the market. At the same time, plummeting crop yields for US farmers prompted a great upsurge in the American demand for guano. As Peru continued to charge the richer countries of the world high prices for its guano and play them off each other to get better deals on guano, the United States took it upon themselves to begin looking elsewhere for alternative sources. Enter the 1856 Guano Islands Act, an act of the US Congress that allowed any entrepreneurial citizen who so desired to take possession of any unclaimed island for the purposes of mining it for guano. As written in that act,

Whenever any citizen of the United States discovers a deposit of guano on any island, rock, or key, not within the lawful jurisdiction of any other government, and not occupied by the citizens of any other government, and takes peaceable possession thereof, and occupies the same, such island, rock, or key may, at the discretion of the President, be considered as appertaining to the United States.

The discoverer, or his assigns, being citizens of the United States, may be allowed, at the pleasure of Congress, the exclusive right of occupying such island, rocks, or keys, for the purpose of obtaining guano, and of selling and delivering the same to citizens of the United States, to be used therein, and may be allowed to charge and receive for every ton thereof delivered alongside a vessel, in proper tubs, within reach of ship’s tackle, a sum not exceeding $8 per ton for the best quality, or $4 for every ton taken while in its native place of deposit.

With the discovery of so many small unclaimed Pacific islands with guano deposits, the United States was able to register 59 different islands under the Guano Islands Act within the first ten years of the act; the first expansion of the United States off of the North American continent and into the realm of international colonisation; a major change in policy for the previously isolationist country. By the end of the 1860s, the United States had acquired Alaska from Russia, followed in the 1890s by the annexation of Hawaii and the acquiring of the Philippines, Puerto Rico and Guam as a result of the Spanish-American War. While the guano boom only lasted half-a-century before it was supplanted by newer artificial fertilisers (not before the habitats of millions of seabirds were destroyed due to mining), the amount of land and water controlled by the Americans as a result of the Guano Islands Act ensured was massive. Many of those islands came into play during the Second World War when the US used its Pacific territories as a large series of military bases.


A map of the United States Minor Outlying Islands in the Pacific, referred to by the US government as the United States Pacific Island Wildlife Refuges.

One of the keys of the act was that for the first time, the United States were now allowed to take possession of lands it had no intention of incorporating into the federal union, creating the new category of insular areas (the category includes not just islands taken under the Guano Islands Act, but all United States possessions outside the union, such as Puerto Rico, Guam, American Samoa, and the Northern Mariana Islands) – they are US possessions, not US territory. Of the dozens and dozens of islands claimed under the act, only ten remain under US jurisdiction today; most having been given to Pacific countries such as Kiribati and Tuvalu. Two islands have since been incorporated into Hawaii and American Samoa, while the other eight remain under the jurisdiction of the Office of Insular Affairs. One (Navassa Island), lies in the Caribbean; the other seven are spread throughout the central Pacific.Together, they are known as the United States Minor Outlying Islands, although the term has been abandoned recently by the US government in favour of the United States Pacific Island Wildlife Refuges. The lands themselves are now protected areas managed by the Fish and Wildlife Services, but the Office of Insular Affairs manages the offshore territories. This is the true benefit for the United States, for they now have right to the 12-mile territorial zone and 200-mile exclusive economic zone (EEZ) around each and every islands, giving the US control of millions of square kilometres of exploitable seafloor under its jurisdiction. Because of these islands, the United States possesses 11 351 000 km2 of ocean floor, the largest EEZ on the planet.


A map of the United States’ exclusive economic zone (EEZ). Source: Lasunncty, Used under the Creative Commons Attribution-ShareAlike 3.0 License.

Further Reading

Bashman, H.(2002). A primer on the Guano Islands Act of 1856. How Appealing, 24 December 2002. Available at Accessed 16 December 2010.

Jensen, B. (2001). Poop Dreams. Baltimore City Paper, 21 February 2001. Available at Accessed 16 December 2010.

Mithra, S. (n.d.). What is Guano? wiseGEEK. Available at Accessed 16 December 2010.

Najafi, S. and C. Duffy Burnett (2010). Islands and the Law: An Interview with Christina Duffy Burnett. Cabinet 38. Available at Accessed 16 December 2010.

Nugent, C. (2006). The Guano Islands Act of 1856 – Congress’s Bird Poop Law. HubPages. Available at Accessed 16 December 2010.

Office of Insular Affairs (2007). Acquisition of Insular Areas. 1 January 2007. Available at Accessed 16 December 2010.

Rosenberg, M. (n.d.). Guano Island Act: Allows U.S. Possession of Islands Containing Bird Droppings. Geography. Available at Accessed 16 December 2010.

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