The poster child for de-extinction is the passenger pigeon. The first European visitors to North America saw flocks so huge that they darkened the skies from horizon to horizon. Even in the 19th century, when the pigeons were starting to decline, observers estimated over a billion birds in some flocks. A market hunter with a shotgun could kill 50 or 100 with a single shot. The combined weight of the pigeons could bring down giant tree limbs with a sound like cannon fire. Yet the last passenger pigeon, a bird named Martha, died in the Cincinnati Zoo in 1914.

What happened? The easy answer is that it was probably a combination of forest loss and market hunting. But in the late 1800s, there were still thousands of pigeons left, some in protected areas, so why didn’t any populations rebound? The likely answer is that the birds, which laid only one egg each, needed hundreds of thousands of other pigeons around to properly mate and nest, and to overwhelm their predators. This is a well-known phenomenon, which biologists call the Allee Effect.

So the de-extinction experts are going to try to revive one the most social birds in history, at great cost, and will bring back four, five or even 50? Even though they might go extinct again?