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

Abstract

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.

Prey availability and foraging activity by tundra‐nesting sea ducks: Strong preference for specific wetland types

“…Chironomidae larvae comprised the largest and most consistent fraction of available relative biomass density (67%-85% in cores, and 50%-74% in sweeps; Table 3), with no other single taxon comprising ≥20% of total biomass density. This pattern of relative total biomass differed from the pattern in diets of adult female sea ducks (Miller et al, 2022). Insect larvae (mostly chironomids) comprised only 4%-14% of protein sources for egg production but comprised 48%-90% of invertebrate biomass in cores and 47%-82% in sweeps (Table 3).…”

“…Our sampling was inadequate to quantify the biomass density of oligochaetes, which stable isotope studies later revealed could be important prey for sea ducks (14%-52% of the diet for the four species studied here, Miller et al, 2022). Oligochaetes vary widely in their relative size and are often considered meiofauna; thus, they are not sampled well by a 0.5-mm sieve which we used as the accepted standard for macroinvertebrate sampling (Higgins & Thiel, 1988;Hummon, 1981;Nalepa & Robertson, 1981).…”

“…Samples were frozen for use in stable isotope studies (Miller et al, 2022;Plesh et al, 2023) et al, 2013). To avoid confounding wetland area with total prey biomass, we opted instead to use invertebrate biomass density, defined as mg C per unit area or volume.…”

“…Cross‐seasonal effects, such as nutrient limitation, may have population‐level impacts on sea ducks (Alisauskas & Devink, 2015 ). On the Arctic Coastal Plain of Alaska, recent studies indicate that sea ducks arriving after a winter at sea rely heavily on invertebrate prey in tundra wetlands to fuel the costs of egg‐laying and incubation (Miller et al, 2022 ). However, the biomass of different invertebrate foods varies among wetland types (Plesh et al, 2023 ), and climate warming is substantially altering the availability of different wetland types on the landscape (Andresen & Lougheed, 2015 ).…”

“…We studied four species of sea ducks of international conservation concern (BirdLife International, 2022a , 2022b , 2022c , 2022d ; USFWS, 2019 , 2021 ) which breed sympatrically in northern Alaska: long‐tailed ducks ( Clangula hyemalis , LTDU) and Steller's ( Polysticta stelleri , STEI), spectacled ( Somateria fischeri ; SPEI), and king ( S. spectabilis , KIEI) eiders. In northern Alaska, foods in tundra wetlands comprised most proteins used for egg production, and at least half of proteins used by females to sustain themselves throughout incubation, with the remainder coming from stored reserves derived from marine foods (Miller et al, 2022 ). In fact, females of some species (Steller's and common eiders S. mollissima ) are known to forego breeding locally in some years, a decision that for common eiders was based in part on female body condition before nest initiation (Coulson, 2010 ; Quakenbush et al, 2004 ).…”