National Geographic : 2018 Mar
46 national geographic • march 2018 weighing less than one gram that continuously records light levels. Because sunrise and sunset times change with location, scientists can ana- lyze the data to trace the path a bird has taken. The study by Hallworth and his colleagues, still ongoing, will enable them to determine precisely where the songbird spends the winter months. “We know that the bird migrates to South Amer- ica, but we have yet to find out where,” he says. Such efforts underscore how far we’ve come in our ability to track bird migrations. Until the early 19th century, theories to explain the disappear- ance of bird populations for part of the year were rather fanciful. Aristotle believed that some birds hibernated or transformed into other species. In medieval Europe the explanation for the appear- ance of barnacle geese in the winter was that they grew on trees. An English minister theorized in the 17th century that they flew to the moon. The most striking evidence that birds were migrating came in 1822, when a hunter in Germany shot Center in Washington, D.C., listened for calls of the Connecticut warbler—a songbird with a yel- low breast and striking white eye-rings. When Hallworth and his fellow researchers spotted a male they had tagged with an electronic device, they worked swiftly to drape a fine net between two trees. Hallworth placed a speaker behind the net, connecting it with a cable to his phone. Hid- ing behind a tree, he played a recording of a male warbler’s song. It was a ruse to lure the warbler to see whether a competitor had entered its territo- ry. Sure enough, the tagged male flew into the net. Extricating the bird, Hallworth gently re- moved the tag on its back—a geolocating device A pair of sandhill cranes performs a mating dance at the Bosque del Apache National Wildlife Refuge in New Mexico. The preserve was created in 1939 largely to protect critical habitat used by these sandhill cranes, which breed in the Rocky Mountains and winter in the southwestern United States and Mexico. THE YEAR OF THE BIRD National Geographic is partnering with the National Audubon Society, BirdLife International, and the Cornell Lab of Ornithology to celebrate the centennial of the Migratory Bird Treaty Act. Watch for more stories, books, and events throughout the year.