Arrival time on breeding grounds can have fitness consequences for migratory birds, with individuals arriving earlier often being able to secure higher quality resources. Determining the arrival time of individuals is thus of interest in many ecological studies.

In most field situations it is very difficult to estimate when exactly birds arrive. If birds arrive on breeding grounds after migrating in an environment that differs isotopically, it is possible to use stable carbon isotope ratios in blood to assess the arrival time of birds to a new habitat.

This approach relies on the well described fact that the carbon isotope ratio in blood approaches a new equilibrium value after a diet switch. The change in blood isotope ratios follows an exponential decay function, and this function can be used to solve for the time when the diet switch occurred if the old and the new diet isotope ratios are known.

We collected blood samples from King Eiders shortly after their arrival on breeding grounds in June, and separated blood samples into plasma and red blood cells (RBC) using a portable centrifuge. We used breast muscle isotope ratios of birds killed by subsistence hunters in Barrow to characterize the marine diet, and used the lowest plasma carbon isotope ratio to characterize the diet consumed by King Eiders on the tundra.