Migratory shorebird stop-over sites: Habitat and prey associations in Oregon estuaries

Background/Question/Methods

Migratory shorebirds rely on stop-over sites, primarily estuaries, to refuel during migration.  Loss of stop-over sites is a primary threat to shorebird populations on the West Coast of the United States (e.g. Calidris alpine arcticola/pacifica, C. mauri).   Conservation and research has focused on the largest of these stop-over sites; however, smaller estuaries also host thousands of migratory shorebirds.  Furthermore, the reasons for stop-over site selection are largely unknown.  Estuarine inter-tidal microhabitats are non-uniform and both abiotic and biotic factors may serve as predictors of whether an abundance of shorebirds will use a given site.  In order to identify factors that may predict site selection, we measured shorebird abundance, habitat characteristics, and food resources – invertebrates and a newly considered source, biofilm --  within two Oregon estuaries during the fall migration period.   Invertebrates were assessed with sediment cores to 3 and 6cm, biofilm with measurement of surface Chl A and ash-free dry mass.  We used generalized linear models to identify the best predictors of where shorebirds foraged.  Additionally, we compiled shorebird abundance data from estuaries throughout the Pacific Northwest to identify broader scale predictors of stop-over site usage.   

Results/Conclusions

Shorebird density, considered over the broader-range, did not correlate with estuary size, but densities tended to be consistent over years.  These patterns indicate that site selection is non-random, and suggest that characteristics other than simply estuary size will best describe site usage on a per-unit size basis.  In the two focus estuaries, shorebird abundance was an order of magnitude higher at the site with greater invertebrate abundance and higher Chl A, but no difference was found in surface ash-free dry mass.  Within estuaries, abundance of biofilm food resources was patchy and did not consistently predict shorebird abundance micro-habitat selection.   These results suggest that overall quality of food resources is important in stop-over site selection, but may not effectively track the finer scale movements of these birds.