In 1532, in the city of Cajamarca, Peru, Spanish conquistador Francisco Pizarro and a group of Europeans took the Inca ruler Atahualpa hostage, setting the stage for the fall of the Inca Empire.

Before this fateful attack, Pizarro’s brother, Pedro Pizarro, made a curious observation: other than the Inca himself, the Lord of Chincha was the only person at Cajamarca carried on a litter, a carrying platform.

Why did the Lord of Chincha occupy such a high position in Inca society? In our new study published in PLOS One, we find evidence for a surprising potential source of power and influence: bird poo.

Chincha, in southern Peru, is one of several river valleys along the desert coast fed by Andean highland waters, which have long been key to irrigation agriculture. About 25 kilometres out to sea are the Chincha Islands, with the largest guano deposits in the Pacific.

Seabird guano, or excrement, is a highly potent organic fertiliser. Compared to terrestrial manures such as cow dung, guano contains vastly more nitrogen and phosphorus, which are essential for plant growth.

On the Peruvian coast, the Humboldt/Peru ocean current creates rich fisheries. These fisheries support massive seabird colonies that roost on the rocky offshore islands.

Thanks to the dry, nearly rainless climate, the seabird guano doesn’t wash away, but continues to pile up until many meters tall. This unique........

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