Using archival light-level tags to describe the migration corridors and wintering ranges of Denali’s Arctic Warblers

The map on the left shows the median location estimates for two adult male Arctic Warblers from summer to spring. The right panels are modeled space use maps during fall migration (top: b, c) and winter (bottom: b, c).

The color of the line and space use maps are the same among individuals (green = 1760-53520 tagged in 2016 and blue = 1780-52921 tagged in 2018). Arctic Warbler photo courtesy of Alan Schmierer.

The Arctic Warbler (Phylloscopus borealis) is a tiny migratory songbird that, along with a handful of other songbird species, has expanded its breeding range from eastern Asia into Alaska. The genus Phylloscopus is in the Family Phylloscopidae, the Leaf Warblers. This Family includes nearly 80 species that nest across Asia and Europe. As of 2022, the Arctic Warbler was the only member of the genus Phylloscopus documented as a breeder in North America.

In Denali National Park and Preserve, Alaska (Denali), adult Arctic Warblers return to their breeding ranges by early June. Over the next few weeks, they court, mate, and lay eggs in their nests that they’ve built on the ground primarily in areas dominated by tall shrub willows along streams. Their nesting season draws to a close by mid-August and they depart for their wintering ranges by late August and early September. Like many of the migratory songbirds that nest in Alaska, Arctic Warblers only spend a small portion of the year in Alaska. During the rest of the year, they are either migrating or on their wintering ranges. But, unlike the other migratory warblers that nest in Denali who are members of the Family Parulidae (the Wood-Warblers) and who winter at lower latitudes in North America, Central, and South America, Arctic Warblers winter in southeast Asia. As part of the NPS Critical Connections Program, launched in 2015 with the goal of describing the annual cycle movements of migratory birds, scientists used data provided by tiny tracking tags known as archival light-level geolocators to quantify the timing of migration, migratory routes, and wintering areas of two adult male Arctic Warblers from their nesting areas in Denali.

The study results, albeit based on a tiny sample size, provided new insight on the migration corridors and wintering areas of Arctic Warblers. For example, the two tagged Arctic Warblers flew to the northwest coast of Alaska when they started autumn migration and flew over the Chukchi Sea on their way to Asia. This suggests that an autumn migration orientation consistent with the great circle migratory pattern as suggested by Alerstam et al. (2008). The shortest route between two points on Earth’s surface is along the great circle, or orthodrome; by using great circle routes, birds save both time and energy during migration. Further, all previous evidence suggested that Arctic Warblers primarily wintered in the Philippines. However, one of the tagged birds may have wintered on the remote island of Palau which is about 890 km east of the Philippines. This suggests that the wintering range of Arctic Warblers that nest in Alaska maybe be larger than previously known. The paper concludes with a series of recommendations for future studies of Arctic Warblers across their annual range.

The first documentation of the Nearctic–Paleotropical migratory route of the Arctic Warbler

Abstract

The Arctic Warbler (Phylloscopus borealis) is a cryptically plumed songbird with an uncommon Nearctic–Paleotropical migratory strategy. Using light-level geolocators, we provide the first documentation of the migratory routes and wintering locations of two territorial adult male Arctic Warblers from Denali National Park and Preserve, Alaska. After accounting for position estimation uncertainties and biases, we found that both individuals departed their breeding grounds in early September, stopped over in southeastern Russia and China during autumn migration, then wintered in the Philippines and the island of Palau. Our documentation of Arctic Warbler wintering on Palau suggests that additional study is needed to document their wintering range. Our study provides hitherto unknown information on stopover and wintering locations for Arctic Warblers and indicates that this species may migrate further overwater than previously thought.

Adams, E. M., I.J. Stenhouse, A. T. Gilbert, J. Boelsma, G. Gress, C. S. Weidensaul, C. Grigsby, E. J. Williams, L. Phillips, and C. L. McIntyre. 2022. The first documentation of the Nearctic-Paleotropical migratory route of the Arctic Warbler. Ecology and Evolution 12(9): e9223.