Summer Shoreline, Part 2 - American Oystercatchers

An adult American oystercatcher

Early in August this year I made a startling sighting on the coastline of Halibut Point. It came at a time when I was at least somewhat prepared for vivid coloration patterns by the through-migrating Ruddy turnstones. The new bird's long stout bill was proportioned like the broadsword of a medieval gladiator audaciously painted red-orange to advertise its lethal potency. A red ring that surrounded the yellow iris and dotted pupil emphasized the eye disc that stared eerily out of the black head above the bill. A human being decked out like that would provoke ambivalence. Does it stand there in jest, deviltry, or mysterious logic? Just the kind of presence a monarch might value to keep court life from getting stale.

A juvenile beside Herring gulls

Unlike various other shorebirds of my acquaintance the oystercatcher scarcely resembled a sandpiper. It made me think of an African stork. It was chunky and about the size of a crow.

This bird species is interesting enough to have attracted admirers, researchers, and advocates into a flock called the American Oystercatcher Working Group (AOWG). Their website is the only reference I've found to apply the wonderful descriptor 'pied'. It calls to mind patchy-patterned mustangs running wild on Chincoteague Island in Virginia which is in the central part of the oystercatcher's range.

Oystercatchers typically gravitate to sandy or gravelly beaches where they can probe for submerged food at the tide line. When it locates a shellfish with open valves, an oystercatcher quickly inserts its bill to sever the adductor chain that holds the halves together. It then extracts and consumes the soft parts.

Less commonly oystercatchers visit rocky shores such as Halibut Point. They pick individual mussels from a clump, hammer them to break through the shell, once again severing the adductor muscle to open the halves and consume the soft parts of the shellfish.

Their large size and white wing band make oystercatchers conspicuous in flight. They are also very vocal when alerted to danger, so you may hear them before you see them. Most likely they will see you first.

The species was hunted to near extinction in the 19th century for plumage and eggs. It has recovered significantly since the passage of the Migratory Bird Treaty Act in 1918 and has begun returning to its former range from refuges in the mid-Atlantic states. Oystercatchers reached coastal Massachusetts in 1969 and are occasionally seen as far north as Nova Scotia.

Adult (L) and juvenile (R)

Local birders have observed oystercatchers in the last few years trying to nest on the offshore islands around Cape Ann, where there are few if any mammalian predators. Gulls, however, take their toll on newborns.

The photograph confirms that oystercatchers are successfully breeding in Massachusetts. As the juveniles reach full size they closely resemble adults, with slightly duller plumage and grayer bills. Their eyes have not yet developed the bright orbital rings.

The AOWG offers some interesting observations about the birds' survival adaptations:

-Their counter-shaded plumage blends in surprisingly well in feeding habitat, particularly mussel and oyster beds, as well as on breeding habitat.

-The young can be heard calling from within the egg two days before they hatch.

-When eggs are present, at the approach of predators or humans, the tending adult slips off nest, usually unseen, while the intruder is still some distance away. It walks rapidly away some distance before taking flight. The bird then exhibits distraction behavior, circling back over the intruder in “butterfly” flight giving distress calls. It often adopts a mock sleeping attitude or mock brooding in an exposed location. In this posture it may allow predatory birds such as gulls to approach and rarely even touch it. Once displaced, the oystercatcher will run some distance, then resume mock brooding posture again.

- Precocial chicks are able to stand upright and run short distances within hours of hatching. They respond to parent alarm calls by running for cover, then lying immobile. Often they will not move until picked up. As fledglings they can escape gulls and peregrine falcons by diving under water, propelling themselves with their wings.

By late summer the oystercatchers begin their return to wintering grounds on the southern shoreline of the United States. They are a bellwether species of healthy coastal environments. We can hope that they will find our region increasingly appealing.