Unraveling migratory connectivity of two European diving ducks: a stable isotope approach

Quantifying the strength of migratory connectivity or the degree to which breeding and wintering populations are connected is crucial for conservation and management of migratory birds. Using stable hydrogen isotope ratios of secondary flight feathers ( δ 2 H f ), we assessed the breeding origins/locations of 666 common pochards ( Aythya ferina ) and 190 tufted ducks ( A. fuligula ) captured in France during the 2005–2006 and 2008–2009 wintering seasons. Isotopic values of common pochards wintering on Grand-lieu Lake (western France) displayed a marked bimodal spatial distribution supportive of strong contributions of both northern (60–65° of latitude) and central European (45–55° of latitude) origins. According to the temporal distribution of δ 2 H f values, the presence of common pochards of northern origin reached a maximum at the peak of the wintering season (December–January). In contrast, tufted duck did not display a clear bimodal pattern in δ 2 H f , with individuals displaying a more northerly origin (60–70° of latitude), likely of Russian and Scandinavian origin. First-year individuals had more northern origins than adults, suggesting that either isotopic discrimination differed among age groups or, more probably, that some adults may undergo moult at latitudes lower than that of their breeding ground. Tufted ducks with northerly δ 2 H f values were observed early in the season in individuals sampled in eastern France and late in the season in those sampled in western France. The effects of winter severity on the distribution of individuals over the wintering range were supported for the tufted duck with more northerly values recorded during harsh winters. Management implications of these results are discussed.