Marjorie Powell is a member of the Golden Gate Audubon Society.
Your Neighbor, the Black Oystercatcher
One of the shorebirds seen in small numbers throughout the year along the Alameda shoreline, black oystercatchers, have all-black bodies, bright yellow eyes with red circles around them, a bright orange-red bill and pink legs. Males and females have the same coloring.
They are medium-sized birds, 17 to 21 inches tall with wing spans of 35 inches (about three feet). Their black bodies provide camouflage as the birds scavenge for food on the rocks at the shoreline, but their bright red bills signal their presence and, once seen, they are easy to follow until they disappear behind a large rock.
Look for them on all of Alameda’s rocky shorelines; they’ve been seen year-round from Seaplane Lagoon to the Elsie Roemer Bird Sanctuary and also on the estuary near the ferry terminal. The best places to view them are Crab Cove and Ballena Bay, where they have been seen nesting.
Black oystercatchers mate for life and occupy the same breeding territory each year. They maintain their pair bonds by engaging in displays throughout the year, walking in pairs while calling loudly, posturing and sometimes flying together. The bird’s call is a high-pitched, loud, fast whistle or shriek that sounds like the word “quick.”
Their nests, well above the high-tide line, are crevices in rocks or scrapes on gravel spots, lined with pebbles or shells. The birds lay two or three pale, beige eggs, with brownish spots. Both parents incubate (sit on) the eggs to keep them warm and protect them from predators.
The chicks hatch in late May or early June, can walk shortly after hatching but remain near the nest, where both parents protect and feed them. The young birds learn to fly by late July or early August, but continue to be fed by the parents for several weeks after that.
Large hawks will eat adult oystercatchers as well as chicks, and both raccoon and skunks will eat their eggs. An adult sitting on eggs may move off the nest if disturbed, leaving the eggs unprotected.
In spite of their name, black oystercatchers feed largely on mussels, crabs, urchins, limpets and marine worms. They often look for mussels at the edge of the water, where the mussels open their shells to feed on tiny organisms in the water. The birds stab their bills into open mussels, gut the muscle that holds the two shells together and eat the mussels without removing the shell from the rocks. Occasionally a black oystercatcher hammers a mussel or other shellfish against a rock to break open a closed shell.
In late summer the adult black oystercatchers molt, or replace worn feathers with new ones. While black oystercatchers that breed in Alaska move slightly south for the winter, those that breed in Alameda stay here all year, although some birds may leave their breeding territory to move locally.
While black oystercatchers are found all along the West Coast, American oystercatchers live along the East Coast and the Gulf of Mexico. They have black heads, brown backs, white bellies and light pink legs, but the same orange-red bills and yellow eyes with red eye-rings.
Because black oystercatchers nest on rocky shores and feed on marine invertebrates, they depend on both adequate rocky shoreline above the highest tides and waters clean enough to support shellfish and marine worms.
Never very numerous, experts estimate their numbers between 12,000 and 18,000 on the West Coast. Their population seems to have risen slightly throughout California over the past 15 years. Their total population is limited by their unique needs — rocky shoreline with clean water and limited human disturbance during spring and summer months.
Their continued presence along Alameda’s shoreline testifies to Alameda’s clean waters but also highlights the need to avoid toxic spills as well as plastic pollution in the city’s marine habitat.
To learn more about efforts to protect and restore bird-safe shoreline through the San Francisco Bay Area, visit the Golden Gate Audubon Society website at www.goldengateaudubon.org.