There weren’t supposed to be any old-growth forests left on Cebu, a lush island in the heart of the Philippines—and with the trees’ absence, no Cebu Flowerpeckers, either. For decades, biologists had written off the stocky songbirds as casualties of rampant deforestation after a 1949 survey found no habitat remaining. But when a biologist scoured the landmass in 1992, he found a pair of the elusive birds, whose males’ black-and-white bodies splotched with primary colors recall a Mondrian painting. The bird’s story proved an instructive, if painful, lesson. “If we didn’t make that assumption in the 1950s [that the Cebu Flowerpecker was extinct], there would be much more forest than there is now,” says Stuart Butchart, chief scientist at BirdLife International. Scientists named the blunder the ‘Romeo error,’ for just as Shakespeare’s Montague took Juliet for dead, ornithologists assumed the bird was a goner, and in doing so failed to protect its shrinking...