In 1967, under a precursor of the Endangered Species Act, 75 animal species became the first to receive federal protection as endangered. Some members of that freshman class, like the Bald Eagle, have since soared off the list after stunning recoveries. Others, like the California Condor, have made significant strides but remain fragile and listed. Somewhere in the middle perches the Kirtland’s Warbler. After bottoming out at 167 breeding pairs in 1974 and again in 1987, the spunky songbird’s numbers have grown steadily, thanks to rigorous, hands-on management. Its population—always naturally small—blew past the recovery goal of 1,000 pairs back in 2001, and today totals more than 2,300 pairs. Given that remarkable rebound, it’s no surprise that the U.S. Fish and Wildlife Service (FWS) last year proposed to take the species off the endangered list, and is expected to make that delisting official this summer or fall. But that doesn’t mean the hard work of...