Guano (via Spanish, ultimately from the Quechua wanu) is the excrement of seabirds, cave-dwelling bats, pinnipeds, or (in English usage) birds more generally. As a manure, guano is a highly effective fertilizer due to its exceptionally high content of nitrogen, phosphate, and potassium, three nutrients essential for plant growth. The nineteenth-century guano trade played a pivotal role in the development of modern input-intensive farming practices and inspired the formal colonization of remote bird islands in many parts of the world. During the twentieth century, guano-producing birds became an important target of conservation programs and influenced the development of environmental consciousness. Today, guano is increasingly sought after by organic farmers.
Seabird guano consists of nitrogen-rich ammonium oxalate and urate, phosphates, as well as some earth salts and impurities. Unleached guano from favored locales, such as the Chincha Islands off the coast of Peru, typically contains 8 to 16 percent nitrogen (the majority of which is uric acid), 8 to 12 percent equivalent phosphoric acid, and 2 to 3 percent equivalent potash. The fresh excrement of insect-eating bats can have nitrogen levels approximating that of seabird guano and high phosphate levels, but bat guano typically has less fertilizing value than seabird guano from arid areas because its nitrogen content is more easily leached within cave environments.
The word "guano" originates from the Andean indigenous language Quechua, which in its original usage refers to any form of dung used as an agricultural fertilizer. Archaeological evidence suggests that Andean peoples have collected guano from small islands and points located off the desert coast of Peru for use as a soil amendment for well over 1,500 years. Spanish colonial documents suggest that the rulers of the Inca Empire assigned great value to guano, restricted access to it, and punished any disturbance of the birds with death. The Guanay Cormorant has historically been the most abundant and important producer of guano. Other important guano producing species off the coast of Peru are the Peruvian Pelican and the Peruvian Booby.
In November 1802, Alexander von Humboldt first encountered guano and began investigating its fertilizing properties at Callao in Peru, and his subsequent writings on this topic made the subject well known in Europe. During the guano boom of the nineteenth century, the vast majority of seabird guano was harvested from Peruvian guano islands, but large quantities were also exported from the Caribbean, atolls in the Central Pacific, and islands off the coast of Namibia, Oman, Patagonia, and Baja California. At that time, massive deposits of guano existed on some islands, in some cases more than 50 m deep.  In this context the United States passed the Guano Islands Act in 1856, which gave U.S. citizens discovering a source of guano on an unclaimed island exclusive rights to the deposits. Nine of these islands are still officially U.S. territories. Control over guano played a central role in the Chincha Islands War (1864–1866) between Spain and a Peruvian-Chilean alliance. Indentured workers from China played an important role in guano harvest. The first group of 79 Chinese workers arrived in Peru in 1849, by the time that trade ended a quarter of a century later, over 100,000 of their fellow countrymen had been imported. There is no documentary evidence that enslaved Pacific Islanders participated in guano mining. 
After 1870, the use of Peruvian guano as a fertilizer was eclipsed by saltpeter in the form of caliche extraction from the interior of the Atacama Desert, not far from the guano areas. During the War of the Pacific (1879–1883) Chile seized much of the guano as well as Peru's nitrate-producing area, enabling its national treasury to grow by 900% between 1879 and 1902 thanks to taxes coming from the newly acquired lands. Contrary to popular belief, seabird guano does not have high concentrations of nitrates, and was never important to the production of explosives; bat and cave-bird deposits have been processed to produce gunpowder, however. High-grade rock phosphate deposits on Nauru, Banaba Island, Christmas Island (Indian Ocean), and other raised atolls were not produced by marine birds.
Since 1909, when the Peruvian government took over guano extraction for use by Peru farmers, the industry has relied on production by living populations of marine birds. U.S. ornithologists Robert Cushman Murphy and William Vogt promoted the Peruvian industry internationally as a supreme example of wildlife conservation, while also drawing attention to its vulnerability to the El Niño phenomenon. South Africa independently developed its own guano industry based on sustained-yield production from marine birds during this period, as well. Both industries eventually collapsed due to pressure from overfishing. The importance of guano deposits to agriculture elsewhere in the world faded after 1909 when Fritz Haber developed the Haber-Bosch process of industrial nitrogen fixation, which today generates the ammonia-based fertilizer responsible for sustaining an estimated one-third of the Earth's population.
The ideal type of guano is found in exceptionally dry climates, as rainwater volatilizes and leaches nitrogen-containing ammonia from guano. In order to support large colonies of marine birds and the fish they feed on, these islands must be adjacent to regions of intense marine upwelling, such as those along the eastern boundaries of the Pacific and South Atlantic Oceans.
Bat guano is usually mined in caves and this mining is associated with a corresponding loss of troglobytic biota and diminishing of biodiversity. Guano deposits support a great variety of cave-adapted invertebrates, that rely on bat feces as their sole source of nutrition. In addition to the biological component, deep guano deposits of both kinds contain local paleoclimatic records in strata that have built up over thousands of years, which are unrecoverable once disturbed.
The greatest damage caused by mining to caves with extant guano deposits is to the bat colonies themselves. Bats are highly vulnerable to regular disturbance to their roosts. Some species, such as Phyllonycteris aphylla, have low fat reserves, and will starve to death when regularly disturbed and put into a panic state during their resting period. Many species will drop pups when in panic, with subsequent death, leading to a steady reduction in population. Research in Jamaica has shown that mining for bat guano is directly related to the loss of bat species, associated invertebrates and fungi, and is the greatest threat to bat caves on the island. However, when done with caution, extraction of guano can be done alongside marine bird colonies without causing them significant harm.
In agriculture and gardening guano has a number of uses, including as: soil builder, lawn treatments, fungicide (when fed to plants through the leaves), nematicide (decomposing microbes help control nematodes), and as composting activator (nutrients and microbes speed up decomposition).
Guano in Popular Culture
The setting of Ian Fleming's 1958 installation in the James Bond series Dr. No (novel) is a Caribbean guano island, and the villain dies at the end buried in guano. In the 1995 film Ace Ventura: When Nature Calls, Jim Carrey's character attempts to save an African tribe from being dispossessed of a fortune in bat guano. In Stanley Kubrick's 1964 film Dr. Strangelove, Colonel "Bat" Guano leads an attack on the airbase responsible for sending a nuclear deployment to bomb the Soviet Union.
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The dictionary definition of guano at Wiktionary