The guano islands off the coast of Peru are home to some of the world’s largest seabird colonies. Millions of cormorants, boobies, and pelicans come here to take advantage of the fish-filled Humboldt Current, making for an awesome spectacle. Equally awesome, if smellier, are the mountains of guano they leave behind. As fertilizer, guano has been a significant international commodity for well over a century; buoyed by a robust export market in the mid-1800s, Peruvians enjoyed decades of guano-based peace and prosperity. But the Guano Era, as it is sometimes called, began drawing to a close in 1864, when the Spanish invaded the islands as part of an effort to reassert control over their former colony. War with Chile over guano and nitrate deposits followed in 1879. It was not until 1909, when the Peruvian government nationalized the islands—including Los Checos, seen here—that relative peace descended, ending decades of overexploitation. Guano is still sold as an alternative...