Guano is the accumulated excrement and organic remains of seabirds, bats, and seals, long prized as a high‑analysis natural fertilizer for agriculture because of its concentrations of nitrogen, phosphorus, and potassium, and historically quarried in arid coastal islands and caves where deposits can reach many meters in thickness, especially along the coast of Peru. According to the editors of Encyclopaedia Britannica, high‑quality bird guano typically contains about 11–16% nitrogen, 8–12% phosphoric acid (P2O5 basis), and 2–3% potash (K2O), with bird guano generally richer in nutrients than bat or seal guano. Birds excrete nitrogen mainly as uric acid, which helps explain the high nitrogen content relative to most mammalian manures; laboratory studies show seabird guano nitrogen is largely uric acid with smaller fractions of protein, ammonia, and nitrate. Stable Isotope Biogeochemistry of Seabird Guano Fertilization: Results from Growth Chamber Studies with Maize (Zea mays); Guano | Fertilizer, Nitrogen & Phosphorus | Britannica.

Etymology and definition

The term “guano” in Spanish derives from the Quechua word wánu (wanu), meaning manure or fertilizer; the Real Academia Española records the usage from the 16th century and defines guano primarily as seabird excrement accumulated on the coasts and islands of Peru and northern Chile used as fertilizer. DLE, Real Academia Española.

Formation, distribution, and environmental setting

Massive surface deposits of seabird guano form where large colonies roost in extremely arid, wind‑swept settings that limit leaching—most famously on islands along the cold, upwelling‑driven Humboldt Current of the southeast Pacific. These conditions historically created extraordinary deposits on the Chincha Islands and other Peruvian guano islands. Guano | Britannica; BirdLife DataZone.

Pre‑Columbian and Inca management

Long before global trade, Andean societies used seabird guano to maintain soil fertility in nutrient‑poor Andean fields. Historical accounts indicate the Inca state regulated access to nesting islands, rationed supplies, and imposed severe penalties for disturbing birds during breeding, a rare early example of explicit wildlife protection tied to soil management. Natural History Museum (London); Audubon.

The 19th‑century “Guano Age”

From the 1840s to the early 1870s, Peruvian seabird guano fueled a global fertilizer boom that transformed European and North American agriculture and state finances; historians estimate Peru exported on the order of 10–12 million tons in 1840–1870, and guano became Peru’s single largest source of revenue. BirdLife DataZone; [Guano and the Opening of the Pacific World](book://Gregory T. Cushman|Guano and the Opening of the Pacific World|Cambridge University Press|2013). European and American interest intensified after reports by naturalists such as Alexander von Humboldt circulated in the 19th century, catalyzing an international scramble for island deposits. The New Yorker.

Competition for control of guano sites sparked conflict. Spain’s seizure of the Chincha Islands in 1864 precipitated the Chincha Islands War (also called the Spanish–South American War), underscoring the deposits’ fiscal and strategic value. Environment & Society Portal.

Labor systems and extraction practices

Guano extraction on Peru’s islands relied first on enslaved and convict labor and, after abolition, on contract laborers from China and other regions who often labored under coercive, hazardous conditions documented by contemporaries and historians. [The Great Guano Rush](book://Jimmy M. Skaggs|The Great Guano Rush: Entrepreneurs and American Overseas Expansion|Palgrave Macmillan|1994); Migration Policy Institute. Contemporary reporting also describes modern regulated harvesting crews scraping consolidated deposits and loading barges on several Peruvian islands. Taipei Times (syndicated NYT report).

U.S. expansion and the Guano Islands Act (1856)

The United States authorized private discovery and occupation of guano‑bearing islands under the Guano Islands Act of 1856, codified in Title 48, Chapter 8 of the U.S. Code. Section 1411 permits a discovered guano island “not within the lawful jurisdiction of any other government” to be considered as “appertaining to the United States” at the President’s discretion; section 1415 restricted export to U.S. use and required bonds; section 1419 clarified the right to abandon islands after removal. LII / Cornell Law School; U.S. Government Publishing Office; LII / Cornell Law School. The act left a legacy of scattered U.S. island claims across the Caribbean and Pacific tied to 19th‑century fertilizer supply chains. LII / Cornell Law School.

Bat guano, saltpeter, and gunpowder

In caves, accumulated bat guano can be leached to yield nitrates, historically processed as saltpeter (potassium nitrate) for black powder. U.S. caves such as Mammoth Cave and other “saltpeter caves” were mined intensively in the early 1800s and again during wartime shortages, with preserved vats and workings still visible. National Park Service; NPS Mammoth Cave feature; TSHA Handbook of Texas.

Decline with synthetic nitrogen and shift to phosphates

Demand for naturally accumulated nitrogenous guano waned in the 20th century with the commercialization of the Haber-Bosch process, which fixes atmospheric nitrogen to produce ammonia at industrial scale and underpins modern nitrogen fertilizer. Britannica: Haber–Bosch process. As high‑grade nitrogen deposits dwindled, industry turned to phosphate ores, many derived from ancient guano diagenesis; spectacular but ultimately devastating extraction occurred on Nauru and Banaba (Ocean Island), where phosphatic residua formed from long‑term guano and bedrock interaction. 1911 Encyclopaedia Britannica: Phosphates; Geography of Nauru; Te Ara Encyclopedia of New Zealand.

Modern conservation and management in Peru

Following 19th‑century overexploitation and subsequent collapses during El Niño events and other pressures, Peru instituted rotational harvests and protected areas to reconcile extraction with seabird conservation. In 2009 the government established the national reserve system covering 22 islands/islets and 11 capes (Reserva Nacional Sistema de Islas, Islotes y Puntas Guaneras; 140,833.47 ha) by Supreme Decree 024‑2009‑MINAM, later supported by operational plans coordinating SERNANP and Agro Rural for limited, timed harvests. Peru—Supreme Decree 024‑2009‑MINAM (SERNANP); SERNANP info sheet; UNESCO Tentative List. Field programs (e.g., Punta San Juan) monitor wildlife during harvest campaigns and advise on mitigation to reduce disturbance. Punta San Juan Program; Brookfield Zoo—Sustainable guano harvests. Recent reporting highlights sharp population declines from avian influenza, El Niño, and prey (anchoveta) variability, illustrating ongoing vulnerability of “guano birds.” Reuters; El País.

Ecological roles and nutrient subsidies

Beyond fertilizer markets, seabird guano is a major vector of marine‑derived nutrients onto islands, altering soil chemistry, boosting plant productivity, and subsidizing adjacent marine food webs. Experimental and comparative studies show elevated soil and plant N and P on seabird islands, reflected in enriched δ15N signatures and cascading effects through terrestrial and nearshore marine communities; invasive rat eradication restores seabird nutrient flows and coral reef functioning over decadal scales. [Oecologia](journal://Oecologia|Nutrient fluxes from water to land: seabirds affect plant nutrient status on Gulf of California islands|1999); PLOS ONE; Nature; Current Biology.

Human health considerations

Disturbance or handling of bat and bird droppings can aerosolize fungal spores (notably Histoplasma), posing a risk of histoplasmosis; public health agencies recommend avoiding disturbance, using wetting methods to suppress dust, and employing appropriate respiratory protection or professional remediation for large accumulations. CDC—Reducing Risk for Histoplasmosis; CDC/NIOSH—Controls for Histoplasma; CDC Yellow Book 2024.

Typical uses today

Seabird and bat guanos remain niche inputs in organic agriculture and horticulture, often marketed for phosphorus and slow‑release nitrogen; however, supply is limited and modern agronomy relies primarily on synthetic nitrogen and mined phosphate fertilizers. Britannica: Guano; Britannica: Haber–Bosch process.