A simple calculation was described in the caption to the photo you see here, when that photo appeared in the report “The U.S. empire was built on bird dung” by Sunjata Gupta, in the January 2025 issue of Science News. The caption says:
“From roughly 1845 to 1880, prospectors in Peru (shown here loading up a cart in 1865) dug up 15 million metric tons of seabird guano for fertilizer. To put that number in perspective, it would take New York City residents 37 years to produce that much feces.”