Guest post by Kerry Wilcox, Sanctuary Manager, Richardson Bay Audubon Center & Sanctuary

In the recent couple weeks, an astounding wildlife phenomenon has been occurring on Richardson Bay and other parts of the greater SF Bay. Thousands of gulls, ducks, and grebes descended on the area to take advantage of an annual, but temporary abundance of food:  herring and their eggs.

Each winter, between December and February, Pacific Herring spawn and release millions of eggs on vegetation and other surfaces. Cormorants, pelicans, Western Grebes, Clark’s Grebes, Red-breasted Mergansers and Forster’s Terns all follow the herring as the fish move about the bay mating and looking for places to deposit their eggs. Once the eggs are laid, Greater and Lesser Scaup, Bufflehead and Surf Scoters are known to feed on the submerged roe, while thousands of gulls representing at least ten species gorge on the exposed roe at the water’s edge.

But this resource is not always available. Not only do the spawn events only take place within a limited time of year, they often are separated by weeks at a time.

So what else do these birds eat during the winter when the roe isn’t so readily available?

The most common species seen during Richardson Bay Audubon’s regular winter waterbird surveys are, in order of abundance: Greater and Lesser Scaup, Ruddy Duck, Bufflehead, Western and Clark’s Grebe, American Coot. All of these species are migratory to some extent, so are mostly gone by April, heading north or east to breed.  Typically, they are used to finding a varied diet throughout the year so are often able to subsist on multiple types of food.

Scaup (and Surf Scoters, which we see fewer of during our surveys, but are relatively common in other parts of SF Bay) for instance, are known to take advantage of the roe when it’s available, but normally eat mussels and clams which they retrieve after diving down and rooting around in the bay sediment. They also eat small invertebrates and some vegetation.

Ruddy Ducks are known to eat mostly aquatic insects, crustaceans and some vegetation.

Bufflehead, the continent’s smallest duck (they actually nests in tree cavities!), eats mostly aquatic insects, crustaceans and mollusks.

Western and Clark’s Grebes mostly dine on fish, but also will capture crustaceans, polychaete worms, and insects

Finally, American Coots are mostly vegetarians and have been noted swallowing entire blades of eelgrass on Richardson Bay; they also will aquatic invertebrates and small vertebrates.