STABLE ISOTOPE ANALYSIS

RESULTS OF EGG ANALYSES

Since 2005 we have collected more than 100 eggs. While there is very large individual variation in the nutrient investment strategy of King Eiders, most of the nutrients in eggs are derived from food consumed on breeding grounds. The graph below shows results of a Bayesian mixing model of 142 King Eider eggs - the posterior distribution indicates the mean range of the proportion of carbon and nitrogen found in yolk and albumen. The black bars indicate nutrients derived from the tundra, white bars represent body reserves. Without foraging on the tundra most King Eiders could not produce a full clutch of 4 eggs!

The isotopic variation of food items found on the tundra is very large. Different females also seem to forage on different foods, leading to a large interindividual variation in isotopic signatures of tissues produced from the local food web. This complicates the analysis of where the nutrients come from. So far, we conclude that King Eiders as a species cannot be categorized as having a single breeding strategy. There are individuals showing a capital, and individuals showing an income breeding strategy.

In any year we estimate that only half of the females we capture in spring is heavy enough to breed. Many birds may therefore migrate to the breeding grounds without actually even attempting to nest. This should be considered when using aerial censuses to estimate a breeding population.

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