Terns, Skimmers, and a Channeled Whelk along the Foster City Bayfront
Saturday, September 1, a decent low tide drew me to the mudflats along the bayfront along Beach Park Boulevard in Foster City (San Mateo County, CA). It had been several years since I last visited this spot, so I was a bit surprised to see that a seawall had been erected along the entire bayfront here; however, I eventually found stairs that led me through the seawall and down to the adjacent beach. There, a low tide had exposed the shell mounds and mud flats that I remembered from past visits here.
The Foster City bayfront is a good spot to look for those mollusks that have been transported here by ships from other shores, and that have subsequently established themselves here. I quickly came across the empty shells of many Pacific Oysters (Magellana gigas), Japanese Littleneck Clams (Ruditapes philippinarum), Atlantic Ribbed Mussels (Geukensia demissa), and Eastern Mudsnails (Ilyanassa obsoleta) – no surprise, as these invasive species have overrun many other locations around San Francisco Bay. I also found many living Eastern Mudsnails here, literally thousands of them oozing their way across the mudflats, feeding on whatever detritus they found there.
Less common were the Eastern Oyster Drills (Urosalpinx cinerea), a muricid that predates oysters, mussels, and barnacles by drilling a hole in their shells to access the contents inside. I also found a Common Periwinkle (Littorina littorea), clamped down tightly on a muddy slab of broken concrete. The empty valve of a native Olympia Oyster (Ostrea lurida) and a couple of introduced Convex Slipper Snails (Crepidula convexa) were interesting finds, but the best find of all was the shell of a Channeled Whelk (Busycotypus canaliculatus) that I found partially buried in the mud. This large, predatory marine snail was introduced from the East Coast of the United State 80 or 90 years ago, and it can grow to around 20 cm in length. The specimen that I found wasn’t quite that large – around 8 cm long.
As I continued exploring the mudflats along the Foster City bayfront, the tide turned and began to rise steadily. By mid-day, it had inundated many of the mudflats, bring the water line closer to shore. This, in turn, attracted quite a few shorebirds, and soon I found myself observing dozens of Willets (Tringa semipalmata), Marbled Godwits (Limosa fedoa), Black-bellied Plovers (Pluvialis squatarola), Long-billed Dowitchers (Limnodromus scolopaceus), and Least Sandpipers (Calidris minutilla) skittering across the mudflats in search of their next meal. As the tide continued rising, seabirds appeared – first, some California Gulls (Larus californicus), followed by Forster’s and Elegant Terns (Sterna forsteri and Thalasseus elegans), which were then followed by Black Skimmers (Rhynchops niger).
Eventually, around 25 Black Skimmers found their way to the Foster City bayfront, where they would periodically take off, wing their way across the sky briefly, and then glide down to a new roosting spot. From a single colony of about 2 dozen birds that first appeared in the South Bay in the 1990s, several colonies have now established themselves around the bay. It was nice to see them doing well.
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