Can intertidal resources alone support wintering shorebirds? Stable isotope analyses of Dunlin Diet in the Skagit River Delta

David Lank
Simon Fraser University
$7,553

Shorebirds rely on both marine intertidal and adjacent terrestrial habitats, forming an ecological link between the two. Shorebirds use high tide roosts on land, but also may feed in agricultural fields, which  have replaced natural salt marsh in some coastal areas. The Puget Sound/Georgia Basin supports over 120,000 shorebirds during the 8-10 month non-breeding season, with the largest concentrations found in the Fraser and Skagit River Deltas. This study enhances an ongoing intensive investigation in the Skagit River Delta of intertidal and terrestrial habitat use by radio-tracked Dunlin, Calidris alpina, the most numerous wintering shorebird in the region. Adding to information on habitat usage (“where birds are”), we will determine the proportion of their diet derived from intertidal versus terrestrial sources (“where birds eat”). We will measure this by taking advantage of natural differences between marine and terrestrial environments in the proportions of stable isotope ratios of nitrogen (isotopes 14 versus 15) and carbon (isotopes 12 versus 13) proportions. By examining the ratios of these isotopes in small blood samples taken from Dunlin, we can determine the proportion of their diet during the previous several weeks that derived from marine versus terrestrial food. In the Fraser River Delta, this method shows that agricultural land adjacent to intertidal areas contributes about a third of the diet of for Dunlin. A similar patterns may or may not occur in the Skagit, and the Dunlin population size supported by each estuary may depend on the relative availability of terrestrial as well as intertidal resources. The results will be made available to land use planners currently discussing management of the Skagit Delta.