The brilliant object sits on display in a Viennese museum—and Mexico’s been wanting it back for decades.IN 1878, THE AUSTRIAN GEOLOGIST and explorer Ferdinand von Hochstetter went prospecting in the hills above Innsbruck. He wasn’t looking for gold or minerals. Rather, he needed exhibits for a newly founded Museum of Natural History in Vienna, of which he had just been named the director. He found what he was looking for in a dusty drawer of the Renaissance-built Ambras Castle—a magnificent piece of feather work, tucked away in a case together with assorted objects from North America, China, and the Sunda...