The black oystercatcher (Haematopus bachmani) with its black body and bright orange bill is a familiar sight on Kodiak Island beaches, but it is one of the least abundant shorebird species in North America. The total world population of oystercatchers is believed to be less than 11,000 birds, but oystercatchers never have been accurately counted, so their population size is unknown. It is also not known whether their population is stable, increasing or decreasing. When the Exxon-Valdez oil spill directly killed a large number of oystercatchers and damaged the intertidal area where black oystercatchers feed, biologists realized how little we know, not only about the population size of these birds, but also about their biology and behavior. We don’t know how long they live, how old they are when they first mate, or what their migratory habits are. Black oystercatchers have been identified as a species of high concern throughout their range in the U.S. and Canada, and federal and state agencies are now working together to learn more about these fascinating birds.
Black oystercatchers range from Baja California to the western Aleutian Islands. Over 65% of the world population resides in Alaska, and more than 1700 oystercatchers live on Kodiak Island.
The black oystercatcher is a large shorebird, approximately 15 inches (38 centimeters) in length. It has a long, heavy, bright orange-red bill, yellow eyes encircled by orange rings, pink legs, and black plumage. its dark feathers often make it hard to see against the black rocks in the intertidal zone, but its loud shrill call announces its presence. In addition to its loud wheep-wheep call, it also has a softer hew-hew-hew call it uses when it is alarmed.
Black oystercatchers live in the upper end of the intertidal zone. They live and nest near an available food source, and if possible, they live near mussel beds, their food of choice. They are territorial during the nesting season, but they often aggregate in groups of tens to hundreds in the winter months. Kodiak Island is the only documented area in Alaska that supports large winter aggregations of black oystercatchers. Winter flocks of twenty to three hundred birds have been counted in the Kodiak Harbor, as well as in Kalsin Bay, Cape Chiniak, Uganik Bay, and Uyak Bay.
Black oystercatchers forage on low-sloping gravel or rock beaches, where prey is abundant. They eat mussels when they are available but also eat other intertidal creatures such as limpets, chitons, crabs, barnacles, clams, and other small animals. An oystercatcher uses one of two methods to eat mussels and clams. If it finds a bivalve with a partially-opened shell, it jabs its bill into the opening, cuts the muscles that hold the shells closed, and consumes the animal. If the shells of the bivalve are closed when the oystercatcher finds it, the bird hammers on the shell to break it open.
Black oystercatchers nest on the shoreline just above the high-tide mark. They may nest on bare rock, sand, gravel, tufts of grass, or among logs. They often nest on small islands, where they are better protected from predators. Nests are built by both parents and are simply a shallow, circular depression lined with shell fragments, rock flakes, or pebbles. Pairs often build more than one nest in their territory, and then the female chooses which nest to use.
Biologists believe black oystercatchers mate for life, and they return to the same nesting territory year after year. In Alaska, they arrive at their nesting sites in March and leave in September. A female lays one to three eggs. The eggs are pale buff or olive and are spotted and marked with brown and black. Both parents incubate the eggs for 24 to 29 days. When the chicks hatch, they are covered with down and stay near the nest at first. Parents take turns guarding the chicks and procuring food. When the chicks are a little older, they follow their parents to the feeding areas, and the parents feed them there. Chicks can fly when they are approximately five weeks old, and they slowly begin to feed themselves. Biologists are not sure how old a black oystercatcher must be before it is sexually mature and can reproduce, but limited evidence suggests they may not be able to reproduce until they are five-years-old. Biologists know black oystercatchers are long-lived birds, but there is little data on how long they live. Some banded birds have lived nearly 16 years.
Next week I’ll tell you more about recent and current research on black oystercatchers and why these birds are considered an indicator species. Don’t forget to sign up for my Mystery Newsletter.