Combining stable hydrogen (δ2H) isotopes and geolocation to assign Scaly-sided Mergansers to moult river catchments

Scaly-sided Mergansers Mergus squamatus breed on freshwater rivers in Far East Russia, Korea, and China, wintering on similar habitat in China and Korea, but information on their post-breeding moulting habitats remains elusive. We combined analysis of stable hydrogen isotope ratios (delta H-2) in flight feathers from nesting females equipped with geolocators to test whether we could correctly identify their use of moulting rivers (which show a strong north-south gradient in river water delta H-2 characteristics) based on feather delta H-2 values. The results are the first ever to demonstrate a strong positive correlation (r(2) = 0.91) between measured river catchment water delta H-2 and feather delta H-2 from birds of known moulting location (from geolocation) in an avian piscivorous species. Furthermore, our delta H-2 results overwhelmingly supported previous determinations based on feather delta C-13 and delta N-15 measurements from the same individuals confirming that most Scaly-sided Mergansers of both sexes moulted on freshwater, although four non-breeding and failed breeding females (out of 21) and one male (out of six) apparently undertook moult migration to brackish and marine waters. The single case where the delta H-2 results contradicted previous isotopic evidence was likely due to birds eating migratory fish of marine provenance that migrate up freshwater rivers. These results confirm the potential power of feather delta H-2 to assign piscivorous birds to specific river catchment moult sites and the utility of using multiple stable isotopes to assign birds to moult habitat and location in potentially complex estuarine and brackish situations or where migratory prey may be used by birds in freshwater habitats.