Validating quantitative fatty acid signature analysis to estimate diets of spectacled and Steller's eiders (Somateria fischeri and Polysticta stelleri).

Fatty acid (FA) signature analysis has been used to study foraging ecology and food webs in marine ecosystems. This powerful method provides information about diets over an extended time period (e.g., 2-4 weeks), rather than just the most recent meal as with most traditional approaches. Using consumer FA signatures, along with a comprehensive database of diet FA signatures, and accounting for consumer FA metabolism, it is possible to estimate the proportions of diet items in the consumer's diet using quantitative FA signature analysis (QFASA). However, before applying QFASA to free-ranging populations, ideally, controlled feeding studies are performed to determine FA deposition and turnover characteristics. We conducted feeding experiments to validate QFASA in captive spectacled eiders (Somateria fischeri) and Steller's eiders (Polysticta stelleri) as a minimally invasive method for studying the diets of these threatened species. We determined FA deposition in eider adipose tissue relative to long-term diet, and developed calibration coefficients (CCs) to account for eider lipid metabolism. Using these CCs with subsequent diet trials, QFASA accurately indicated diet and diet switches. QFASA estimates also indicated that turnover of dietary FAs was not complete by 21 or 29 days, and confirmed that diets could be estimated over an extended period of >29 days. Thus, our understanding of diet can be backtracked to more than a month in captive feeding eiders. We conclude that applying QFASA techniques to eiders and other birds in the wild has the potential to provide valuable information about their diets at various life history stages.