This September 1 is the 100th anniversary of a landmark event in the history of biodiversity. On that day in 1914, at about one o’clock in the afternoon, Martha – the last surviving passenger pigeon – died at the Cincinnati Zoo. It is extraordinary to know with virtual certainty the day and hour when a species ceases to be a living entity. And it was a stunning development because less than half a century earlier, the passenger pigeon had been the most abundant bird in North America, if not the world.

As late as the 1860s, passenger pigeons had likely numbered in the billions, and their population was neither evenly distributed across the landscape

nor in any way subtle. These birds had a propensity for forming huge aggregations that are difficult to imagine today. John James Audubon, America’s best-known student of birds, recorded a flight of passenger pigeons along the Ohio River in Kentucky that eclipsed the sun for three days. Other accounts, written over the course of three centuries and in several languages, testify to the birds darkening the sky for hours at a time over the major cities of the eastern third of the United States and Canada.