One hundred million years ago, the sky was filled with birds unlike those seen today, many with long, streamerlike tail feathers. Now, paleontologists have found examples of these paired feathers preserved in exquisite detail in 31 pieces of Cretaceous amber from Myanmar. The rare 3D preservation reveals the feathers’ structure is completely different from that of modern feathers—and hints that they may have been defensive decoys to foil predators. Such tail streamers—in some cases longer than the bodies—have been observed in early bird fossils from China for several decades, in particular, the 125-million-year-old Confuciusornis sanctus. They may also be present...