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.