Bird droppings are a nuisance for most people, but in Peru they have been a closely guarded treasure since pre- ­Columbian times.

Guano, a gentler word for dung, is one of the few words in English derived from Quechua, the language of the Incas. The Incas used guano harvested from islands that dot Peru’s 1,500-mile coastline as fertilizer. They fiercely guarded the source. And execution was the ultimate punishment for anyone who disturbed the sea birds or the islands where they deposited dung.

Execution is off the table today, but not much else has changed. The Peruvian government maintains strict control over the islands, which are part of a coastal protected reserve. Guano remains a highly prized fertilizer, and Peru is at the top of the field thanks to the gigantic quantity of oily anchovies that guano birds feast on — which makes their dung valuable — and the unusual nature of Peru’s desert coast. It never rains, so the guano just piles up.

Guano is harvested much the same way it was hundreds of years ago, with a squadron of workers manually scraping, sifting and bagging it. The government, through a division of the Agriculture and Irrigation Ministry, selects about 400 men each year to work eight months as harvesters. More than 60 percent of the workers return from one harvest to the next, and many are relatives. The work is limited to one or two islands, sometimes three, in each campaign.

The islands are chosen after an evaluation by biologists to guarantee that no birds are nesting. The harvest target this year is about 20,000 tons of guano. The yearly campaign was just past the halfway mark in June.

The work is grueling, as is evident in images by Alvaro Ybarra Zavala, a Spanish photographer who was allowed to visit Asia Island in November 2017. Teams use picks, shovels and brooms to loosen the guano, which is several yards thick in some spots. Machinery cannot be used because of the rugged and dung-slick terrain, and because it would spook the birds.

Workers earn 2,400 soles (about $750) a month, more than double the average national monthly income of about $300. They would earn much less in the farming villages where they are recruited — including Cajamarca, the country’s poorest state. They generally work eight-hour shifts and receive meals and health insurance.

The majority of the guano stays in Peru and is sold locally at a subsidized rate to help boost production for small-scale farmers. About 25 percent of it is sold at market value either in Peru or abroad.

The sacks of guano that harvesters carry on their shoulders weigh up to 50 pounds. Alvaro Ybarra Zavala/For The Washington Post

Workers cover their faces against swirling dust and the stench of the droppings. Alvaro Ybarra Zavala/For The Washington Post

Harvesting methods haven’t changed in hundreds of years. Workers stave off the monotony by rotating shifts. Alvaro Ybarra Zavala/For The Washington Post

The guano, sometimes yards deep, must be separated from remnants of the rocky surface before it can be processed. Alvaro Ybarra Zavala/For The Washington Post

Even on an overcast day, the sun and constant wind require workers to take care in how they dress. Alvaro Ybarra Zavala/For The Washington Post

As one worker pours, his peers sift through the guano to separate it before bagging. Alvaro Ybarra Zavala/For The Washington Post

The effects of the work on harvesters are unmistakable. Weariness is but one element. Alvaro Ybarra Zavala/For The Washington Post

With machinery banned on the islands to protect the birds, the industry operates on brawn. Alvaro Ybarra Zavala/For The Washington Post

Birds that live on the islands feast on oily anchovies from the nearby waters, producing large amounts of waste that can serve as fertilizer. Alvaro Ybarra Zavala/For The Washington Post

Peru’s cold waters are a perfect breeding ground for the anchovies in the guano birds’ diet. They’re good for a dip, too. Alvaro Ybarra Zavala/For The Washington Post

The islands are empty of plant life. They are home only to birds and their many tons of dung. Alvaro Ybarra Zavala/For The Washington Post

A guano worker talks by cellphone to his family at the end of the day. Alvaro Ybarra Zavala/For The Washington Post

Photo edit by Olivier Laurent. Design and development by Sam Schumacher.