Seabamirum/Flickr
The United States used to belong to passenger pigeons. The highly successful birds traversed the continent in flocks of hundreds of millions, impervious to predators from their sheer numbers alone.
Yet today no passenger pigeon lives. What happened in a span of just half a century?
As technological innovations brought European settlers deeper into North America in the mid-19th century, over-hunting and habitat destruction drove the birds to a swift extinction.
Check out these photos of the passenger pigeon's tragic demise - and a high-tech effort that just might bring them back.