As spring unfolds in North America, the days lengthen and warm, flowers push out of thawing soils, and green leaves bud on branches. Bees get busy, and birds court mates, build nests, and raise young. As any winter-hardened birder knows, many of those birds have been absent, passing harsh, cold northern months in warmer southern climates. Some have flown thousands of miles to arrive to their breeding grounds in the same place, at about the same time, as last spring and the spring before that. Why birds undertake these seasonal journeys seems at first intuitive: They seek to improve their chances, and their offspring's chances, of survival. But how bird migration at first evolved has, in fact, long been a topic that scientists have debated. Researchers don’t know enough about ancient birds and their closest relatives from many millions of years ago to determine when the first avian ancestors began migrating. Existing evidence goes back at least hundreds of thousands...