The last known photo of the slender-billed curlew, a grayish-brown migratory waterbird, was taken in February 1995 at Merja Zerga, on Morocco’s Atlantic coast. There will likely never be another one.

A mysterious migratory bird is the first of its kind to be declared extinct. Here’s what that means for its friends in the bird world. While other migratory birds have become extinct, those commonly ...