A multidisciplinary study reveals that nutrient-rich seabird guano was a key driver of agricultural productivity and sociopolitical expansion in ancient coastal Peru – long before the rise of the Inca Empire.
Published in PLOS ONE, the study presents some of the strongest archaeological and biochemical evidence that the accumulated droppings of marine birds played a defining role in the development of the Chincha Kingdom, one of the most powerful pre-Hispanic societies on Peru’s southern Pacific coast, which emerged in the Chincha Valley.
Situated in one of the driest regions on Earth, the Chincha Valley posed a significant problem for sustainable agriculture. Irrigation from the valley’s river systems enabled crop cultivation, but the sandy, nutrient-poor soils lost fertility quickly.
The answer, according to archaeologist Dr. Jacob L. Bongers and colleagues, was a chain of islands colonised by seabirds known as the Chincha Islands, some 25 km from the coast.
The Chincha Islands are home to enormous colonies of seabirds — including guanay cormorants, Peruvian boobies and Peruvian pelicans — whose guano deposits have long been treasured as a potent organic fertiliser.
Researchers conducted isotope analyses on 35 maize (corn) cobs dating from AD 1150 to 1675, as well as on seabird remains recovered from nearby sites.
The study found that the N isotope ratios (δ¹⁵N) in the maize samples were elevated, consistent with marine enrichment, far beyond what would be expected from natural soil conditions alone. These patterns clearly support the intentional application of seabird guano as fertiliser, supplementing irrigation and greatly enhancing maize yields.
“The isotopic evidence suggests that Indigenous farmers were deliberately importing and applying marine fertiliser to enhance agricultural production at least by AD 1250 and likely earlier,” the paper states. This practice mirrors earlier findings of guano use in northern Chile but represents the strongest evidence yet of its role in Peru’s southern coast.
These findings are confirmed in historical sources from the colonial period. Chroniclers describe coastal communities sailing to offshore islands in rafts to collect seabird droppings, and colonial administrators acknowledged the value of guano as both a commodity and a strategic resource. Some early colonial accounts even record that the Inca Empire imposed strict laws governing the collection of guano, underscoring its importance in pre-Hispanic and early colonial agronomy and trade.
These increased agricultural outputs, the study’s authors contend, did more than feed a greater proportion of people — they facilitated economic diversification, population growth and the expansion of the Chincha Kingdom’s political power.
Their abundant maize helped the Chincha bolster merchant relationships, specialised production, and long-distance trade along the Pacific coast, thereby reinforcing their role as a regional force.
This finding not only reshapes our understanding of pre-Inca agriculture but also highlights the broader impact of environmental resources on the development of social complexity in ancient societies.
By linking a seemingly mundane biological by-product to the rise of a major prehistoric polity, the study serves as a starting point for rethinking the nature of interactions between natural ecosystems, human innovation, and social and political development in the Andean world.
Header Image Credit : iStock
Sources : PLOS ONE – https://doi.org/10.1371/journal.pone.0341263
