Nathan Cooper, Smithsonian Migratory Bird Center

Watch tiny geolocator map rare bird’s round-trip migration

Fifty years ago, the Kirtland’s warbler seemed to be on its last legs. Fewer than 400 of these Michigan songbirds, which weigh less than an empty soda can and are only slightly taller, were left, so the species, Setophaga kirtlandii, became one of the first beneficiaries of the U.S. Endangered Species Act. Today, its population has increased 10-fold, making it a poster child for conservationists and bird lovers alike. That improvement came from planting more jack pines, which the bird needs to nest in, in its summer home in upper Michigan. Even so, only about 40% of the birds survive the annual migration to the Bahamas and back. To learn more about this journey, researchers outfitted 50 warblers with tiny geolocators (weighing just 0.5 grams—at the time, the smallest ever developed for an animal), which recorded the light intensity every 2 minutes, data indicative of sunrise and sunset that can be used to calculate latitude and longitude. In the fall, the warblers first headed east over Ontario to the coast, then turned southward to the Bahamas, the team reports this week in the Journal of Avian Biology. The warblers took a different route in the spring, flying straight west and stopping off in Florida for a week or so before turning north toward Michigan. Each way, they travel about 4500 kilometers in about 16 days. This was the first time anyone has tracked this species over an entire year, and though not much was learned about what it does en route, the locations of stopovers is a starting point for improving its protection, the researchers say.