December in Moscow, and the temperature drops under 15 degrees below zero. The radiators in the bar have grown cold, so I sit in a thick coat and gloves drinking vodka while I ponder the fossil birds. The year is 2001, and Evgeny N. Kurochkin of the Russian Academy of Sciences and I have just spent hours at the paleontology museum as part of our effort to survey all the avian fossils ever collected in Mongolia by joint Soviet-Mongolian expeditions. Among the remains is a wing unearthed in the Gobi Desert in 1987. Compared with the spectacularly preserved dinosaur skeletons in the museum’s collections, this tiny wing—its delicate bones jumbled and crushed—is decidedly unglamorous. But it offers a strong hint that a widely held view of bird evolution is wrong.