Many long-distance migratory bird species seem to retain the winter quarters their ancestors used during the last glacial maximum. Post-glacial colonization following the retreat of the ice sheets have then resulted in apparent suboptimal migration routes, since the expanding populations have failed to use more nearby, alternative and climatically suitable, wintering areas. The bluethroat subspecies Cyanecula svecica svecia occurs in the northern areas of the Palearctic from the Atlantic to Alaska. Because it is monotypic and lacks population structure in mitochondrial DNA it is assumed to have colonized its present range from one glacial refuge population. Geolocator tracks from birds in its western range have shown that these migrates to the Indian subcontinent following a route west of the Himalaya. In the present study we fit geolocators of bluethroats breeding in their eastern range (Chukotka) to test whether they also use a route west of the Himalaya to the same wintering grounds as their European conspecifics, or shortcut east of the Himalaya. We retrieved one of the 30 bluethroats tagged which clearly showed that it migrated east of the Himalaya to a wintering area tentatively located in Myanmar. By assuming that this bird is representative for the migration of eastern Cyanecula s. svecia, we can reject our initial hypothesis of a common migration route of all Cyanecula s. svecia. The different routes and wintering areas of western and eastern Cyanecula s. svecia implicate the presence of a migratory divide somewhere north of the Himalaya, and further that the populations were in different refuge populations during the last glaciation.

The data from the geolocator was extracted by Migrate Technology LTD. We used the R version 4.1.1 (R Core Team 2021) and the package GeoLight 2.0.0 (Lisovski and Hahn 2012) to process the light data to obtain coordinates. Great circle distances were calculated with R package geosphere 1.5–10 (Hijmans 2019). We set the threshold of light intensity to > 1 lx. With GeoLight, we used a “Loess filter” and a K value 1 to remove extreme outliers. Further, we omitted all longitudes west of 0° and latitudes south of 0°. The logger was set to start data collection on the 15th of July 2018 and continued to record until 13th of March 2019. Due to the bush-dwelling lifestyle of Bluethroats, the majority of the collected data, even after heavy filtering, was of poor quality except for the month of February. When using the data restricted to February, which should reflect a stationary period of the winter quarter, a Hill-Ekström calibration provided a sun elevation angle of − 1.9 and corresponded to latitudes matching northern Mongolia, which for climatic reasons is an unlikely winter site. We then increased the sun elevation angle to + 3 for latitude estimates, to include the closest suitable winter sites in southern Asia (Supplementary Table 1). Because the longitudes are not as sensitive to data quality or choice of sun elevation angle, we estimated the longitudinal positions also for the autumn migration period.