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Biofilm is on the Kids’ Menu, and Other Lessons from the Western Sandpipers of San Francisco Bay

USGS scientists are studying what western sandpipers in San Francisco Bay eat to fuel up for their migration. This research can inform conservation and management efforts for this tiny shorebird.

Western sandpiper, with a grey back and heat and white underside, standing in shallow water, one foot up
A Western sandpiper in San Francisco Bay.

The western sandpiper (Calidris mauri) is a small shorebird that weighs only about 30 grams. That’s a little less than a slice of bread! During the winter, western sandpipers can be found foraging on mudflats all along the Pacific Coast from British Columbia to Peru. In the spring, western sandpipers migrate north to their breeding area in Alaska. That is a long way for these little birds to travel, and these migratory flights require a lot of energy!

San Francisco Bay is both a winter home for many western sandpipers as well as a stopover location for those that winter further south. USGS scientists at the Western Ecological Research Center have been studying how western sandpipers in San Francisco Bay fuel up for their long migrations. They wanted to find out what the western sandpipers were eating, and how the food they were eating varied with age and sex.

Satellite image map of San Francisco Bay, with a star marking Dumbarton Shoal in South San Francisco Bay
A satellite image of SF Bay showing the study location for researching Western sandpiper diets, at the Dumbarton Shoal in South San Francisco Bay

How can you tell what a bird is eating? Well, you can watch them eat, or you can look at what’s in their guts. But there’s a less time consuming, less invasive strategy, called stable isotope analysis, that the USGS scientists used for the sandpipers. Scientists use carbon and nitrogen isotopes to study animal diets. Isotopes are simply varied forms of the elements found in nature. For example, the carbon-13 isotope is a little heavier than the carbon-12 isotope because it has an extra neutron.

Different types of plants, animals, or microorganisms have different ratios of carbon and nitrogen isotopes in their bodies because of the foods they eat or the way their bodies work. When a sandpiper feeds, the carbon and nitrogen in their food becomes carbon and nitrogen in their bodies, and the isotope ratios in their food can now be detected in the sandpiper. As a result, the isotope ratios in western sandpiper blood can provide hints to what they have eaten.

Using this method, the USGS researchers learned about the diets of the SF Bay sandpipers. It turns out that in January and February, western sandpipers eat similar things regardless of age or sex: mostly a mix of tiny invertebrates, including bivalve clams and polychaete worms. But in April, at the onset of the spring migration, birds of different ages and sexes had different diets. For example, adult females ate more polychaete worms than any other group, and juveniles, especially males, ate more microphytobenthos (tiny photosynthetic organisms) and biofilm than adults. Biofilm is a slimy mix of microphytobenthos, bacteria, and detritus.

The difference between seasons is likely driven by the need to fuel up for migration—just like you might eat a big pasta dinner before a big game. Biofilm is energy rich and more abundant in the spring. With lots and lots of sandpipers and other bird species competing to find high energy foods in the Bay during spring (almost half a million shorebirds visit San Francisco Bay each year!), birds with different sized bills may start specializing, eating more of the food to which they have easy access. Female adult sandpipers have longer bills than males and juveniles, which may explain why they eat more polychaete worms, which are buried deeper in the sediment than some other invertebrates. Juveniles might also have a different diet because they have less foraging experience and lower social status—you might say that biofilm is on the kids’ menu (though adults eat it, too).

Understanding what western sandpipers eat helps scientists develop population models that inform wildlife management and conservation efforts. Even a slimy biofilm can make a difference to the health of the western sandpiper!

Learn more about USGS shorebird and food web research:

Food Web and Invertebrate Ecology

Avian Ecology and Multi-Species Habitat Use in Pacific Coast Estuaries

Western sandpiper, with food items--worms, crustaceans, tiny bivalves, green slime--in circles surrounding the sandpiper image
A western sandpiper (Calidris mauri) with some of the food items available in San Francisco Bay, CA, USA. Food items pictured include amphipods and copepods (small crustceans), bivalves, oligochaete worms, and biofilm.


Figure showing the makeup of sandpiper diets in winter and early spring, by age and sex.
Diets of western sandpipers (Calidris mauri) of different ages and sexes during mid-winter (Jan/Feb) and spring migration (Apr) in San Francisco Bay, CA, USA determined with stable isotopes. Each bar represents the diet of one bird. Ad. F. = adult females, Ad. M. = adult males, Juv. F. = juvenile females, Juv. M. = juvenile males. Figure from Hall et al. 2021.


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