For much of the year, migratory songbirds were treated as temporary visitors to the tropics. They arrived from the north, spent the winter, and departed again. Ornithologists paid more attention to their journeys and breeding grounds than to the months they passed farther south. The birds were often described as transients, moving through tropical forests without becoming part of them. What they did there, and how firmly they occupied those places, remained poorly understood. Years of catching, banding, and watching them suggested a different account. In the forests of southern Mexico, individual birds returned to familiar patches and defended feeding territories. They were regular members of tropical bird communities for months at a time. A bird that appeared to be a passing visitor might return to the same small area year after year. Its life in the tropics was structured, local, and essential to its survival. Migration could not be explained only as an escape from northern winters. Its origins might instead lie in dispersal from warmer regions, as birds gradually extended their ranges toward seasonal food and breeding opportunities. John Hilton Rappole, who died on June 30th, aged 79, pursued this argument for much of his career. It became known as the dispersal theory of bird migration. The theory challenged a long-standing explanation for why birds crossed continents. His commitment to it reflected his approach to science. He relied on sustained field observation, questioned established assumptions, and was prepared to defend an idea over many years. He was less…This article was originally published on Mongabay