Geolocator Tracking and Stable Isotope Analysis Suggest Mixed Migration Strategies in White-Shouldered Starlings ().

The East Asian-Australasian Flyway (EAAF) is one of the least studied flyways, especially for migratory passerines. A better understanding of the migratory behaviour of birds along the EAAF is critical to inform conservation efforts and to improve the understanding of the evolution of migratory behaviour. White-shouldered starlings () breed in southern China and northern Vietnam, with at least some populations migrating to distinct wintering grounds. Using a combination of extrinsic (geolocators) and intrinsic markers (stable isotopes), we tested whether a presumed resident breeding population in Hong Kong is resident or migratory. We discovered that most tagged individuals (7/8) migrated southwest from Hong Kong to winter in Vietnam/Cambodia up to 2013 km from their breeding grounds. We also found evidence of an alternative migration pathway, with one individual migrating northeast to overwinter in southern China. Stable isotope analyses of feathers revealed that the northeast-migrating individual had different δH and δS values than those migrating southeast in that year (but had similar isotope values in the previous year). Based on isotope data alone, a second individual was identified who had distinct δH and δS values between years, suggesting that this individual also switched wintering grounds between the two breeding seasons. Together our results confirm that the Hong Kong breeding population is migratory, and both geolocator and stable isotope data suggest that the Hong Kong population of this species might host at least two different migration strategies, with some individuals switching their wintering grounds between years.