Ancient Peruvian civilisation grew mighty by harvesting guano

A powerful fertilizer made from seabird droppings may have fueled the rise of a Peruvian agricultural kingdom 900 years ago and contributed to its eventual takeover by the Incas.

Chemical analyzes of ancient corncobs from southern Peru show unusually high levels of nitrogen isotopes – clear signs that the plants were fertilized with a mixture of seabird droppings, feathers and carcasses called guano. The findings provide the strongest evidence yet that indigenous Chincha farmers, fishermen and traders harvested this nutrient-rich fertilizer on nearby islands to improve inland crop fields – and strengthen their socio-economic position, says Jacob Bongers of the University of Sydney.

“Privileged access to a crucial resource is a path to power – which the Chincha kingdom had in this case, but the Incas did not,” he says. “Social change may have come from a surprising source: bird droppings. It’s a fascinating story.”

Between 1000 and 1400 AD, the wealthy and densely populated Chincha kingdom controlled one of Peru’s most productive coastal valleys before being incorporated into the Inca Empire in the 15th century.

The Chincha Valley is just 25 kilometers from the Chincha Islands, which are home to large colonies of Peruvian pelicans (Pelecanus thagus), the Peruvian madmen (Sula variegata) and guanay cormorants (Leucocarbo bougainvilliorum), as well as penguins and seagulls. These so-called Guano islands gained international notoriety in the 19th century thanks to the fertilizing power of their bird droppings, largely thanks to their exceptionally high nitrogen content.

The use of guano by the Incas is well documented in early colonial accounts, which describe strict state control over the islands and heavy penalties for harming birds. But until now, scientists lacked solid archaeological evidence that their Chincha predecessors were already exploiting the resource. Many historians have long argued that this was the case – and that access to seabird fertilizer fueled the kingdom’s economic success, Bongers says. Images of seabirds carved into ceremonial objects and depicted on textiles, ceramics, and architectural friezes further suggest that birds had particular importance to the Chincha.

Bongers had collected dozens of ancient corncobs – “perhaps food for the dead” – from Chincha tombs and wondered if they might help solve the mystery.

He teamed up with Emily Milton of the Smithsonian Institution in Washington DC to analyze 35 ears of corn from 14 cemeteries in the Chincha Valley, measuring their carbon and nitrogen isotope ratios. Bongers, Milton and their colleagues also analyzed collagen from 11 ancient seabird bones from the region – including pelicans, boobies, cormorants, a seagull and a penguin – to establish a local isotopic baseline for guano.

Ancient seabird bones had high nitrogen-15 values, typical of seabirds. Many corn cobs had even more extreme nitrogen isotope ratios, a hallmark of seabird guano fertilization.

The results indicate that the Chincha have been using the island’s resources for at least 1,250 years, says Jo Osborn of Texas A&M University.

Guano may have supported the kingdom’s economic expansion and strengthened its bargaining power when it was later incorporated into the Inca empire – with broader implications for how marine fertilizers shaped social change across the Andes, researchers say.

“It makes perfect sense that ancient Peruvians used guano as fertilizer,” says Dan Sandweiss of the University of Maine, who was not involved in the study. “It was a big expedition to get to the islands, but you’re doing this for things of great value!”

Guano from Chincha Island is particularly valuable, probably because of limited rainfall, he says, which allows the nitrogen to remain intact, rather than being washed away. “That Peruvian guano was the real thing.”