When humans disappear from the planet, we could leave behind a fowl legacy. The signature fossil of our current slice of geologic time might be the bones of the estimated 23 billion chickens on Earth at any given time, a population 10 times greater than that of any other bird species, The New York Times reports. By examining the fossil record from the early centuries C.E. until today, researchers noticed an unprecedented change in the size, number, and composition of the chicken bones at the advent of modern factory farming in the mid–20th century, the researchers report today in Royal Society Open Science. Nowhere is human intervention more apparent than in the bones of the broiler chicken, a species bred and fed specifically for mass consumption.