The Critical Connections team tagging Swainson's thrushes in Denali National Park. ©Scott WeidensaulThe Critical Connections project -- underwritten by the National Park Service, the Denali Education Center, Camp Denali/North Face Lodge and other supporters -- is an ongoing effort to trace the migratory routes of the breeding birds of Denali National Park in Alaska, which fan out across nearly two-thirds of globe each year on their travels.

Spearheaded by Dr. Carol McIntyre of Denali National Park, Dr. Iain Stenhouse of the Biodiversity Research Institute and Scott Weidensaul, the project focuses onm Denali's more than 165 species of birds -- passerines, raptors, shorebirds, seabirds and others -- using the latest in tracking technology.

In 2015, the team launched Critical Connections by using tiny geolocators to trace the movements of Swainson's A geolocator, weighing less than a gram, will record this gray-cheeked thrush's migration to Latin America. ©Scott Weidensaulthrushes, which breed in the park's spruce forests and winter in Central and South America, and gray-cheeked thrushes, which inhabit willow thickets in the rolling tundra landscape and travel to northern South America.

The next year we recaptured the returning thrushes to download their stored migration data, allowing us to track their movements as far south as Bolivia and northern Argentina. We have also turned to a fresh suite of birds: hermit thrushes that occupy fast-spreadsing poplar forests in the park; Arctic warblers that winter in the Philippines, Indochina and Malaysia; Wilson's warblers, which migrate to Mexico and Central America; blackpoll warblers that migrate to northern South America; and fox sparrows that travel to the Southeastern U.S.