Wild Side : This strange, hardy bird
The boundless creativity of evolution is best displayed in species that have developed ways to thrive in the face of adversity, such as fire, drought, or toxic environments. Most often, such exceptionally hardy species use their adaptation to gain a competitive advantage: if a plant can resist the harmful effects of salt, for example, it can have space to itself on beaches where most other plants would promptly die. I admire the ruggedness of such survivors, as they turn adversity into advantage, and I'm fascinated by the structures or chemical tricks rendering them immune to conditions that ought to kill them.
Purple sandpipers seem immune to even the harshest winter weather. Click on photo to enlarge. Photo by Tim JohnsonSometimes, though, one comes across a species whose fondness for a difficult setting appears to be wholly gratuitous. Late winter provides a good opportunity to observe one such bird that winters on the Vineyard: the purple sandpiper, a shorebird about the size of a sparrow that makes its living, preposterously, by picking small invertebrates off of surf-swept, weedy rocks. Clambering nonchalantly on a steep, slippery surface, dodging energetic waves of nearly frozen seawater, the rock snipe, as it was once called, ranks among the toughest animals I know.
Purple sandpipers breed in the Arctic on both sides of the Atlantic and, though swift fliers, they are reluctant migrants and winter farther north than any other shorebird. Though never numerous on the Vineyard, purple sandpipers are nonetheless reliably present here throughout the winter, typically arriving in late November and leaving in late March or April. And their very strong habitat preference (and enduring fondness for particular spots) make them quite easy to find. Even isolated boulders around the Island's shoreline can host them, but the extensive rocks around Squibnocket Point (including the ones visible from the beach parking area) produce this species almost invariably.
The jetties across from Waban Park and Farm Pond in Oak Bluffs often hold a purple sandpiper or two, and the birds can sometimes be spotted from the overlook at the Gay Head cliffs. As interesting as its penchant for certain sites is the purple sandpiper's avoidance of other, seemingly suitable places. I've never, for example, seen one on the Vineyard Haven jetties, or on the rip-rap at the foot of East Chop. You'd have to ask a purple sandpiper to explain this.
Two other sandpipers winter here in modest numbers and could be confused with the rock snipe. The sanderling is a very pale bird in winter, short-billed and with black legs, and its preference for sandy shoreline is almost as strong as the purple sandpiper's fondness for rock. The dunlin resembles the purple sandpiper in its size and lengthy bill, and it often forages on breakwaters and cobble. But it has longer black legs than the purple sandpiper, which also has a darker back than the dunlin and sports distinctly yellow or orange legs. Between its leg color and its behavior, the rock snipe is simple to identify.
During the summer, on its high arctic breeding grounds, purple sandpipers behave like normal shorebirds, which is, to be sure, odd enough. This group of birds is characterized by dramatic courtship displays and a tendency to provide very little care for their young, which hatch well-developed and almost self-sufficient. According to published accounts, purple sandpipers nest and feed in a range of wet and dry settings, often a long way from the ocean, sometimes even at fairly high elevations. But in winter, when plenty of perfectly good shorebird habitat is nearly empty of competitors, and when most closely related species have moved much farther south, what do they do?
They cling to the coastline, and are rarely found away from the breaking surf. They cling to steep, slippery rocks along the northern Atlantic coast, constantly chased by breaking waves. They confine themselves to a habitat that is limited in spatial extent and only accessible during part of the tide cycle. Passing up opportunities that it is perfectly well equipped to exploit, the rock snipe instead risks having its brains bashed out by errant waves and insists on feeding in spots that require constant acrobatics.
Often, a bird with highly specialized behavior will have evolved striking physical characteristics as well. For example, oystercatchers (not such distant relatives of the rock snipe) have evolved a blade-like bill that they deploy like an oyster knife on bivalves. But the purple sandpiper's physical adaptations are limited. Its legs are short and stocky, no doubt welcome traits when one is clinging to a wet rock. And when they do get doused by a wave, purple sandpipers can swim adeptly - a quite common ability among shorebirds.
So while their behavior obviously makes sense to the sandpipers themselves, it makes little sense to me. This strange, hardy bird illustrates the inscrutable complexity that makes the wild side so interesting.
