Exploring the diet consumed by sea ducks on breeding grounds is of major interest to researchers and managers. Sea ducks migrating to arctic breeding grounds have to produce eggs shortly after arrival, and for many species it is unknown to what extent diet on breeding grounds contributes to nutrients allocated to eggs. Waterfowl leave eggshell membranes in the nest after chicks hatch, and these membranes can be collected without affecting the nest or the laying female. By analysing isotope ratios in eggshell membranes it is possible to draw inference about the diet consumed by the laying female during egg production.

We analyzed carbon and nitrogen isotope ratios in eggshell membranes of King Eiders and found that isotope ratios did not change during the course of incubation. This makes it possible to use membranes of hatched or depredated eggs to infer what diet has been used for egg production. Membranes can, however, not resolve how much endogenous nutrients were invested into eggs.