Results and Discussion
We recaptured 6 of the 66 Arctic Warblers (9.1%) banded from 2016–18, including 5 of the 42 (11.9%) with attached geolocators. All recaptures occurred within the 101-ha study area; some birds were recaptured multiple times after geolocators were deployed (Appendix B). The distance between capture and recapture locations in subsequent years for all birds averaged 260 m (range = 132–637 m). Two of the five geolocators had useful data to track individuals from their breeding territories to their wintering grounds in Southeast Asia, one of which also extended into the early portion of spring migration.
The warblers departed the breeding grounds in Denali by 8/29 and 9/3 (and 1780-53921 and 1760-53520, respectively) flew northwest, leaving North America from the western edge of the Lisburne Peninsula (Figure 1). They arrived at the wintering grounds by 10/20 and 10/15, respectively. The autumn migration route included overwater flights from Alaska to Russia and from China and Japan into the Philippines and the surrounding Pacific Islands (Figure 1A). Both individuals appeared to linger in southeastern Russia, then again in southeast China and Japan, during fall migration before arriving at their respective wintering grounds (Figure 1B). Their autumn migration orientation is consistent with the great circle migratory pattern reported by Alerstam et al. (2008). The birds traveled approximately 8,500 and 8,700 km to the mean wintering locations and averaged 197 and 181 km/day, respectively. One individual’s wintering locations centered on the island of Palawan in the Philippines, while the other individual’s wintering locations were centered in Palau (Figure 1C). There is no prior evidence of Arctic Warblers wintering on Palau. Both geolocators failed before the birds returned to Alaska, but a portion of spring migration along the western coast of Japan and the Kamchatka Peninsula was documented for one bird (1780-53921).
All previous evidence suggested that Arctic Warblers primarily wintered in the Philippines (Lowther & Sharbough 2020), necessitating some overwater migration when traveling to and from North America. However, if wintering occurs on remote islands like Palau (about 890 km from the Philippines), more or longer overwater flights would be necessary. Such flights are not uncommon for small birds; many songbirds and hummingbirds make a flight of similar length across the Gulf of Mexico annually (Deppe et al. 2015; Weidensaul et al. 2020). However, extending such flights represent increases in mortality risk during migration due to encounters with extreme weather events (Newton 2007). Given the potential for longer overwater flights, the importance of stopover habitat before those legs (e.g., the locations more frequent use in SE Russia, SE China, and Japan) increases as greater fuel loads are required (Vincze et al. 2019).
The geolocators we used in our study are low precision position estimation tools that can be persistently shaded by feathers and vegetation. Bird behavior and environmental conditions can cause pathological errors in the data collection that systematically bias the results (Hahn et al.2020). Thus, these position estimates could be confounded by systemic shading issues. Therefore, our confidence in the Palau wintering location is not high, given the lack of prior evidence of this species wintering there. While our data suggested winter space-use was focused on Palau for one individual, this pattern could be influenced by a systemic bias in sunrise or sunset times. These biases can be quantified in some situations, but both data loggers stopped recording before the birds returned to their recapture locations and prevented us from comparing known locations with estimation locations after the migration was complete. Alternatively, if these data are correct, we have documented a previously unknown wintering location for the species, and the winter range for this species should be reevaluated. Given the lack of eBird data collected on the island, the nondescript plumage ofPhylloscopus warblers, and the potential for range shifts, this overwinter location seems possible. While we found no evidence of our data being systematically biased, the result’s novelty suggests that caution is needed until the Arctic Warbler’s presence can be confirmed on Palau.
This study has documented the migratory pathway and suggests a new wintering location of Arctic Warblers. Geolocators and similar methods that track movements across the full annual cycle in small songbirds continue to be powerful tools in illuminating idiosyncratic movements for poorly studied species, and advancing our understanding of factors affecting the conservation of migrants with complex life histories (Tonra et al. 2019). However, the costs of such studies to the tagged individuals also need to be considered. Our recovery rate of tagged birds was 11.9%, much lower than the 35% resighting rate of color-banded Arctic Warblers at a different study site, 210 km east of our study area (Lowther & Sharbough 2020). Low recapture rates may have resulted from low breeding site fidelity and low survival rate (Ralphet al. 1993; DeSante et al. 2015), lower survival due to a tag effect (Costantini & Møller 2013; Brlík et al. 2020), and our inability to detect tagged birds that had returned to the study area. Furthermore, we noted intra-season movements that indicated large territories or inter-territory movements (average = 235 m, range = 83–610; n = 10 birds) that may have contributed to our low recapture rate. Thus, we believe studies quantifying demography, nesting area fidelity, and within and among breeding season movements need to be conducted before deploying further tracking devices on this species.
The Arctic Warbler breeding range is now much reduced after the taxonomic split (Alstrӧm et al. 2011; Withrow et al.2016), which implies a much smaller population size than was previously thought. Furthermore, there is evidence that the species is responding quickly and negatively to impacts of climate change on the breeding grounds in Alaska (Mizel et al. 2016; Thompson et al.2016). As such, this species’ risk profile has increased, and we recommend expanding current efforts to help understand Arctic Warbler population status and the ecological drivers of population trends across the annual cycle.