The season for kayaking and boating is upon us, and I'm cleaning up my kayak gear in preparation. To help kick off the season, Juneau Audubon will conduct its annual cruises to Berners Bay, on April 18 and May 2. Critter-watchers on the cruise will have their eyes peeled for anything that moves, and one of the featured critters will be the black oystercatcher.
There are several species of oystercatcher in the world, but we have just one in the North Pacific area. The black oystercatcher is a sizable, chunky shorebird with a long, gaudy red bill and incongruously pale pink legs. The plumage is black, tending toward dark brown posteriorly, but the general appearance from the usual viewing distance is black. Close-up, the red eye-ring and yellow iris are striking.
Oystercatchers like to hang out on rocky reefs, which offer good foraging for mussels, limpets, and snails, along with crabs, barnacles, small urchins, and chitons - but very few oysters. Many of our oystercatchers appear to move south along the coast for the winter, but, by early spring, they may be seen in small groups on reefs around here. When the tide is out, they also forage in the intertidal area of rocky and sandy beaches.
As spring progresses, these birds become highly territorial, each pair vigorously defending its nesting area from intruders. Their loud, piping call can be heard during territorial chases, and a softer version is used as a greeting between mates. The territories are centered on a nest site, located above the high tide line on beaches, seldom very far above the highest line of debris tossed up by wave action. Near Juneau, they nest on several of the small islands between Douglas and Point Bridget. The main nesting season is in May and June in our area.
The nest is usually just a shallow scrape in the sand and gravel, lined with rock chips and bits of shell. There are usually two or three eggs in a clutch, each with an assortment of dark spots and blotches on a buffy background. So the nest and eggs are very inconspicuous if the adults happen to be off the nest. If disturbed, an incubating adult leaves the nest and may run a little way and squat down as if incubating in another location. They may repeat this diversionary behavior several times-and it can be successful in diverting the attention of a predator - or a human. It is all too easy for an exploring human to step on the real nest, while diverted by the fake squatting behavior.
Unfortunately for nesting oystercatchers, humans like the same beaches for picnics, camping, and romping around. The birds are known to be sensitive to human disturbance, which appears to be one of the primary causes of nesting failure - along with predators, such as bears, mink, gulls, ravens, and storm surges that can wipe out a nest. At best, only about 50-60 percent of nests are successful. The black oystercatcher is therefore on the "watch list" of conservation concern for Audubon, both nationally and in Alaska.
Oystercatchers are relatively long-lived and do not typically breed until they are about five years old. The eggs are incubated by both adults for about four weeks - or more if disturbed. The downy chicks can walk when about three days old but are often brooded by adults until they are over a week old. As the chicks grow, they begin to follow adults to nearby feeding areas; they begin to feed for themselves when they are about two weeks old, although adults continue to bring food for several months.
I once watched an adult oystercatcher bring a small crab to a young chick that had followed its parent down the beach. The adult put the crab down in front of the chick, which ignored the prey. So the adult poked the crab, picked it up and put it down again, poked it some more, until eventually the chick got the idea and started to peck at the crab itself. It takes young birds about a year to become foragers as proficient as their parents.
The closest I ever got to oystercatchers was on a kayak trip in Glacier Bay in late summer. Several of us were sitting very quietly on the beach, watching whales. As the tide came in, a group of five Black Oystercatchers retreated up the beach toward us. Close inspection revealed that this was a family of two adults and three large chicks. The chicks were distinguishable because their black was rather dingy, the bill was not yet bright red, the eye rings were orange instead of red, and the iris was still dark. As long as we were still, this little family seemed unconcerned by our presence, eventually coming within about 15 feet before gradually moving off along the shore.
Late in summer, oystercatchers begin to congregate; sometimes many families gather on the same reefs for foraging before starting their migration. Portland Island is reported to be a favorite gathering spot.
Mary F. Willson is a retired professor of ecology.
Juneau Empire ©2014. All Rights Reserved.