Use of marine vs. freshwater proteins for egg‐laying and incubation by sea ducks breeding in Arctic tundra

Understanding dietary nutrient sources is fundamental to conserving sensitive species, especially as climate change alters food web dynamics. Migratory species that depend on both marine and terrestrial habitats face unique challenges, as the locations and quality of resources in the two realms may respond quite differently to environmental changes, with potential for spatial and temporal carryover effects. For sea ducks (Mergini) that winter at sea but move inland to breed, body size may determine their capacity to store nutrient reserves for later use in alternative habitats. We assessed ultimate sources of protein for reproduction in four sea duck species in northern Alaska: smaller‐bodied Long‐tailed Ducks and Steller's Eiders (Clangula hyemalis and Polysticta stelleri), and larger‐bodied Spectacled and King Eiders (Somateria fischeri and Somateria spectabilis). To assess the relative use of local freshwater foods vs. marine protein for both egg production and body maintenance of incubating females, we measured stable isotopes of carbon and nitrogen in egg membranes, red blood cells, marine and freshwater invertebrates, and vegetation. For egg production, isotope mixing models indicated that proteinaceous egg membranes of all four species were derived mostly (89%–95%) from freshwater foods on the breeding grounds, with broad individual variation in specific prey types selected by the larger species. For incubation, isotopes in red blood cells indicated that body maintenance of females also relied mainly (87%–91%) on freshwater foods in Long‐tailed Ducks and Steller's Eiders. However, incubating Spectacled and King Eiders obtained only about 60% of their protein from freshwater foods and the remainder from marine‐derived body tissues. The latter strategy allows the larger‐bodied species to incubate almost continuously, whereas the smaller species must take more frequent incubation breaks and generally incur higher rates of predation on eggs. Thus, depending on body size, cross‐seasonal effects of feeding conditions in marine habitats may strongly influence population processes well after the birds move to inland nesting sites. Although conservation programs on land and sea are often researched, planned, and administered by different agencies and organizations, our results emphasize the need to coordinate marine and land‐based efforts for species that integrate conditions across both environments.