Black Oystercatcher Life History

Habitat

Black Oystercatchers spend their entire lives in view of the Pacific Ocean or adjacent bays, in rocky marine habitats that provide both nesting and foraging areas. Nest sites include shelly, gravelly, or sandy beaches and spits as well as treeless rocky headlands or islands. They generally avoid areas with cliffs, which provide limited opportunities for foraging, instead preferring level areas of coastline or sites with a gradual slope. They feed in open mudflats in many areas as well. On occasion they forage on earthworms in open, grassy sites (such as golf courses) adjacent to the ocean. During strong storms, Black Oystercatchers often seek shelter in bays and harbors, away from usual feeding and nesting locations.

Food

Black Oystercatchers eat mostly mollusks and other marine creatures that inhabit the rocky intertidal zone. They are visual hunters. For bivalve mollusks, such as blue mussels, they pry open the shell and quickly cut the adductor muscle, then eat the shellfish and discard the shells. They hunt especially on rising and falling tides. At this phase of the tide, waves periodically wash over the mussels. The bivalves open their shells more often, making it easier for oystercatchers to pry them fully open. Oysters, seldom eaten, are usually punctured near the shell’s hinge, to reveal the adductor muscle. For clams and other bivalves that live in mud or sand, oystercatchers may probe into the substrate or look for signs of their presence, such as siphon holes. For single-shelled mollusks, such as limpets, whelks, and chitons, oystercatchers chisel the animal off the rock using their bill, then turn the prey upside down to consume. They also eat crabs and sea urchins, which they smash and open for the meat, and barnacles, which they twist from inside the shell. They eat bristleworms (polychaetes) and occasionally earthworms by probing into sand, mud, or soil. On rare occasions they hunt Pacific mole crabs on sandy beaches in the same manner. They do not feed at night, as some other oystercatcher species do. Sometimes, they eat small isopods (crustaceans such as woodlice and sea roaches) by plucking them as they race across the rocks. Among their specific prey items are Pacific blue mussel, sea mussel, bay mussel, horse mussel, Pacific littleneck clam, butter clam, granular claw crab, and purple urchins. They also forage on marine organisms that have washed up, such as jellyfish, sea anemones, by-the-wind sailors (hydrozoans), or the spawn of Pacific herring.

Nesting

Nest Placement

Nests are set above high-tide line on rocky shorelines, headlands, or beaches (of shells, pebbles, gravel, cobble, or sand) free of taller vegetation. Nests on islands are usually on smaller islands that lack trees.

Nest Description

Males often make multiple scrapes on the ground. Females select one of these. The nest may be bare or include bits of rocks, pebbles, or shell, carried to the nest by both sexes. Nest size varies, averaging about 8 inches across, 1 inch deep.

Nesting Facts

Behavior

Black Oystercatchers appear to be monogamous in their mating system, and pairs remain together year-round. During the spring, as the breeding season approaches, male and female perform paired flight and walking displays accompanied by much calling. Pairs hold territories and also perform side-by-side walking displays against other oystercatchers at the territorial boundary. Territorial birds may also drive others out by flying at them or chasing them in flight, calling vigorously. Both adults incubate eggs and tend chicks, which can walk shortly after hatching. Adults feed their young and teach them foraging techniques for several months after the young have fledged. After the breeding season, some pairs and young remain in the breeding territory, while others disperse toward richer feeding areas, where they may gather in flocks of dozens or even a few hundred in winter.

Conservation

Black Oystercatchers are fairly common along Pacific shorelines. Partners in Flight estimates a global population of 10,000 breeding birds, rates the species a 15 out of 20 on the Continental Concern Score, and includes it on the Yellow Watch List for species with restricted ranges. Almost the entire population of this species nests in Canada and the United States, with a small population in Baja California, Mexico. Populations of this species have been reduced by feral cats. In Alaska, they are vulnerable to rats and foxes that have been introduced to offshore islands. Like other shorebirds, nesting oystercatchers are sensitive to beach disturbance by humans. Oil spills such as the Exxon Valdez spill in 1989 have caused deaths in this species and lasting damage to its coastal habitat.

Credits

Andres, Brad A. and Gary A. Falxa. (1995). Black Oystercatcher (Haematopus bachmani), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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