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Guano (Spanish from ) is the accumulated excrement of or . Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a lesser extent, sought for the production of gunpowder and other explosive materials.
The 19th-century seabird guano trade played a pivotal role in the development of modern input-intensive farming. The demand for guano spurred the human colonisation of remote bird islands in many parts of the world.
Unsustainable seabird guano mining processes can result in permanent habitat destruction and the loss of millions of seabirds.
Bat guano is found in caves throughout the world. Many cave ecosystems are wholly dependent on bats to provide nutrients via their guano which supports bacteria, Fungus, , and . The loss of bats from a cave can result in the extinction of species that rely on their guano. Sustainability harvesting of bat guano may cause bats to abandon their roost.
Demand for guano rapidly declined after 1910 with the development of the Haber–Bosch process for extracting nitrogen from the atmosphere.
Guano mining continues in Chile with the annual guano production in Chile ranging from 2,091 to 4,601 metric tons per year in the 2014–2023 period.
Seabird guano contains plant nutrients including nitrogen, phosphorus, calcium and potassium.
Bat guano composition varies between species with different diets. Insectivore bats are the only species that congregate in large enough numbers to produce sufficient guano for sustainable harvesting.
Although the first shipments of guano reached Spain as early as 1700, it did not become a popular product in Europe until the 19th century.
The arrival of commercial whaling on the Pacific coast of South America contributed to scaling of its guano industry. Whaling vessels carried consumer goods to Peru such as textiles, flour, and lard; unequal trade meant that ships returning north were often half empty, leaving entrepreneurs in search of profitable goods that could be exported. In 1840, Peruvian politician and entrepreneur negotiated a deal to commercialise guano export among a merchant house in Liverpool, a group of French businessmen, and the Peruvian government. This agreement resulted in the abolition of all preexisting claims to Peruvian guano; thereafter, it was the exclusive resource of the State. By nationalising its guano resources, the Peruvian government could collect royalties on their sale, which became the country's largest source of revenue. Some of this income was used by the State to free its more than 25,000 black slaves and to abolish the head tax on its Indians. This export of guano from Peru to Europe has been suggested as the vehicle that brought a virulent strain of potato blight from the Andean highlands that began the Great Famine of Ireland.
Soon guano was sourced from regions besides Peru. By 1846, of guano had been exported from Ichaboe Island, off the coast of Namibia, and surrounding islands to Great Britain. Guano pirating took off in other regions as well, causing prices to plummet and more consumers to try it. The biggest markets for guano from 1840–1879 were in Great Britain, the Low Countries, Germany, and the United States.
By the late 1860s, it became apparent that Peru's most productive guano site, the Chincha Islands, was nearing depletion. This caused guano mining to shift to other islands farther north and south. Despite this near exhaustion, Peru achieved its greatest ever export of guano in 1870 at more than . Concern of exhaustion was ameliorated by the discovery of a new Peruvian resource: sodium nitrate, also called Chile saltpetre. After 1870, the use of Peruvian guano as a fertiliser was eclipsed by Chile saltpetre in the form of caliche (a sedimentary rock) extraction from the interior of the Atacama Desert, close to the guano areas.
The Guano Age ended with the War of the Pacific (1879–1883), which saw Chilean marines invade coastal Bolivia to claim its guano and saltpetre resources. Knowing that Bolivia and Peru had a mutual defense agreement, Chile mounted a preemptive strike on Peru, resulting in its occupation of the Tarapacá, which included Peru's guano islands. With the Treaty of AncĂłn of 1884, the War of the Pacific ended. Bolivia ceded its entire coastline to Chile, which also gained half of Peru's guano income from the 1880s and its guano islands. The conflict ended with Chilean control over the most valuable nitrogen resources in the world. Chile's national treasury grew by 900% between 1879 and 1902 thanks to taxes coming from the newly acquired lands.
Other countries also used their desire for guano as a reason to expand their empires. The United Kingdom claimed Kiritimati and Malden Island for the British Empire. Others nations that claimed guano islands included Australia, France, German Empire, Hawaiian Kingdom, Japan, and Porfiriato.
From the 1930s, Bat Cave mine in Arizona was used for guano extraction, though it cost more to develop than it was worth. U.S. Guano Corporation bought the property in 1958 and invested $3.5 million to make it operational; actual guano deposits in the cave were 1% of predicted and the mine was abandoned in 1960.
In Australia, the first documented claim on Naracoorte's Bat Cave guano deposits was in 1867. Guano mining in the country remained a localised and small industry. In modern times, bat guano is used in low levels in developed countries. It remains an important resource in developing countries, particularly in Asia. (2025). 9783319252186, Springer International Publishing. ISBN 9783319252186
Similarly, harvesting bat guano in caves was and is manual. In Puerto Rico, cave entrances were enlarged to facilitate access and extraction. Guano was freed from the rocky substrate by explosives. Then, it was shoveled into and removed from the cave. From there, the guano was taken to kilns to dry. The dried guano would then be loaded into sacks, ready for transport via ship. Today, bat guano is usually harvested in the developing world, using "strong backs and shovels".
Despite these policies, the seabird population continued to decline, which was exacerbated by the 1911 El Niño–Southern Oscillation. In 1913, Scottish ornithologist Henry Ogg Forbes authored a report on behalf of the Peruvian Corporation focusing on how human actions harmed the birds and subsequent guano production. Forbes suggested additional policies to conserve the seabirds, including keeping unauthorised visitors a mile away from guano islands at all times, eliminating all the birds' natural predators, maintaining armed patrols on the islands, and decreasing the frequency of harvest on each island to once every three to four years. In 2009, these conservation efforts culminated in the establishment of the Guano Islands, Isles, and Capes National Reserve System, which consists of twenty-two islands and eleven capes. This Reserve System was the first marine protected area in South America, encompassing .
In addition to harming bats by necessitating they find another roost, guano harvesting techniques can ultimately harm human livelihood as well. Harming or killing bats means that less guano will be produced, resulting in unsustainable harvesting practices. In contrast, sustainable harvesting practices do not negatively impact bat colonies nor other cave fauna. The International Union for Conservation of Nature's (IUCN) 2014 recommendations for sustainable guano harvesting include extracting guano when the bats are not present, such as when Animal migration are gone for the season or when non-migratory bats are out foraging at night.
Hundreds or thousands of , especially Native Hawaiians, traveled or were blackbirded to the U.S.-held and Peruvian guano islands for work, including Howland Island, Jarvis Island, and Baker Island. While most Hawaiians were literate, they could usually not read English; the contract they received in their own language lacked key amendments that the English version had. Because of this, the Hawaiian language contract was often missing key information, such as the departure date, the length of the contract, and the name of the company for which they would be working. When they arrived at their destination to begin mining, they learned that both contracts were largely meaningless in terms of work conditions. Instead, their overseer (commonly referred to as a luna), who was usually white, had nearly unlimited power over them. Wages varied from lows of $5/month to highs of $14/month. Native Hawaiian labourers of Jarvis Island referred to the island as Paukeaho, meaning "out of breath" or "exhausted", due to the strain of loading heavy bags of guano onto ships. Pacific Islanders also risked death: one in thirty-six labourers from Honolulu died before completing his contract. Slaves blackbirded from Easter Island in 1862 were repatriated by the Peruvian government in 1863; only twelve of 800 slaves survived the journey.
On Navassa Island, the guano mining company switched from white convicts to largely black labourers after the American Civil War. Black labourers from Baltimore claimed that they were misled into signing contracts with stories of mostly fruit-picking, not guano mining, and "access to beautiful women". Instead, the work was exhausting and punishments were brutal. Labourers were frequently placed in stocks or tied up and dangled in the air. A labour revolt ensued, where the workers attacked their overseers with stones, axes, and even dynamite, killing five overseers.
Although the process for mining guano is mostly the same today, worker conditions have improved. As of 2018, guano miners in Peru made US$750 per month, which is more than twice the average national monthly income of $300. Workers also have health insurance, meals, and eight-hour shifts.
Rabies, which can affect humans who have been bitten by infected mammals including bats, cannot be transmitted through bat guano. A 2011 study of bat guano in the U.S. states of Texas and California recovered no viruses that are pathogenic to humans, nor any close relatives of pathogenic viruses. It is hypothesised that Egyptian fruit bats, which are native to Africa and the Middle East, can spread Marburg virus to each other through contact with infected secretions such as guano, but a 2018 review concluded that more studies are necessary to determine the specific mechanisms of exposure that cause Marburg virus disease in humans. Exposure to guano could be a route of transmission to humans.
As early as in the 18th century there are reports of travellers complaining about the unhealthy air of Arica and Iquique resulting from abundant bird spilling.
Cave ecosystems are often limited by nutrient availability. Bats bring nutrients into these ecosystems via their excretions, however, which are often the dominant energy resource of a cave. Many cave species depend on bat guano for sustenance, directly or indirectly. Because cave-roosting bats are often highly colonial, they can deposit substantial quantities of nutrients into caves. The largest colony of bats in the world at Bracken Cave (about 20 million individuals) deposit of guano into the cave every year. Even smaller colonies have relatively large impacts, with one colony of 3,000 annually depositing of guano into their cave.
Invertebrates inhabit guano piles, including Diptera, , , , , , , silverfish, , harvestmen, , , , , and barklice. The invertebrate communities associated with the guano depends on the bat species' feeding guild: frugivorous bat guano has the greatest invertebrate diversity. Some invertebrates feed directly on the guano, while others consume the fungi that use it as a growth medium. Predators such as spiders depend on guano to support their prey base. Vertebrates consume guano as well, including the bullhead catfish and larvae of the grotto salamander.
Bat guano is integral to the existence of endangered cave fauna. The critically endangered Shelta Cave crayfish feeds on guano and other detritus. The Ozark cavefish, a U.S. federally listed species, also consumes guano. The loss of bats from a cave can result in declines or extinctions of other species that rely on their guano. A 1987 cave flood resulted in the death of its bat colony; the Valdina Farms salamander is now likely extinct as a result.
Bat guano also has a role in shaping caves by making them larger. It has been estimated that 70–95% of the total volume of Gomantong Caves in Borneo is due to biological processes such as guano excretion, as the acidity of the guano weathers the rocky substrate. The presence of high densities of bats in a cave is predicted to cause the erosion of of rock over 30,000 years.
English author Robert Smith Surtees parodied the obsession of wealthy landowners with the "religion of progress" in 1843. In one of his works featuring the character John Jorrocks, Surtees has the character develop an obsession with trying all the latest farming experiments, including guano. In an effort to impress the upper class around him and disguise his low-class origins, Jorrocks references guano in conversation at every chance he can. At one point, he exclaims, "Guano!" along with two other varieties of fertiliser, to which the Duke replies, "I see you understand it all!"
Guano is also the namesake for one of the in RNA and DNA: guanine, a purine base, consisting of a fused pyrimidine-imidazole planar ring system with conjugated double bonds. Guanine was first obtained from guano by , who incorrectly first described it as xanthine, a closely related purine, in 1844. After he was corrected by Einbrodt two years later, Bodo Unger agreed and published it with the new name of "guanine" in 1846.
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