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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.
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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.
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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.