A potent fertilizer derived from seabird droppings likely played a crucial role in the rise of Peru’s agricultural kingdom 900 years ago, contributing to its eventual conquest by the Incas.

Recent chemical analysis of ancient corn cobs from southern Peru revealed elevated nitrogen isotope levels, indicating the maize was fertilized with a nutrient-rich blend of seabird excrement, feathers, and organic matter known as guano. This discovery represents the strongest evidence yet that indigenous Chincha farmers and traders sourced this exceptional fertilizer from nearby islands to enhance their crop yields and elevate their socio-economic status, according to Jacob Bongers at the University of Sydney.

“Access to vital resources can pave the way to power; here, the Chincha kingdom had the upper hand, while the Incas did not,” he states. “Social change may have originated from an unexpected source: bird droppings. It’s quite the intriguing narrative.”

Between 1000 and 1400 AD, the affluent and populous Kingdom of Chincha dominated one of Peru’s most fertile coastal valleys until its eventual incorporation into the Inca Empire in the 15th century.

The Chincha Valley lies just 25 kilometers from the Chincha Islands, home to large colonies of Peruvian pelicans (Pelecanus sagus), Peruvian boobies (Sula variegata), and Guanaius (leucocarbo bougainvilli), as well as penguins and gulls. These islands are known as guano islands. Bird droppings became renowned globally in the 19th century for their remarkable nutritional properties, primarily due to their high nitrogen content.

The use of guano by the Incas is well documented in early colonial records, which describe stringent state regulations governing the islands and substantial penalties for harming seabirds. However, until now, researchers lacked solid archaeological proof that the ancestors of the Chincha were already exploiting this valuable resource. Historians have long suggested that seabird fertilizers significantly bolstered the kingdom’s economic success, Bongers notes. Images of seabirds depicted on ceremonial artifacts, textiles, pottery, and architectural features further indicate that these birds held special significance for the Chincha people.

Bongers and his team gathered numerous ancient corn cobs (possibly offerings for the deceased) from Chincha tombs, hoping to unlock this mystery.

He collaborated with Emily Milton from the Smithsonian Institution, analyzing 35 corn cobs from 14 cemeteries in the Chincha Valley for their carbon and nitrogen isotope ratios. Together with Bongers, Milton and their colleagues also examined collagen from the bones of 11 ancient seabirds from the region, including pelicans, boobies, cormorants, gulls, and penguins, to establish a local isotope baseline for guano.

The bone analysis showed elevated nitrogen-15 levels typical of seabirds, while many corn cobs displayed even higher nitrogen isotope ratios indicative of guano fertilization.

These findings confirm that the Chincha people were utilizing island resources by at least 1250, says Joe Osborne from Texas A&M University.

Guano may have facilitated the economic expansion of the Chincha kingdom and enhanced its bargaining capacity when later absorbed into the Inca Empire, with broader implications for understanding how marine fertilizers influenced social transformations throughout the Andes, the researchers contend.

“It’s logical that ancient Peruvians harnessed guano as fertilizer,” remarks Dan Sandweiss from the University of Maine, who was not involved in the study. “Traveling to the islands for such a valuable resource would certainly have been worthwhile!”

Chincha guano is exceptionally valuable, possibly because its limited rainfall allows the nitrogen to remain largely intact. “This Peruvian guano was indeed a treasure,” he adds.

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