If I asked you to guess what is the most abundant bird on Cape Cod and the Islands this time of the year, which would you choose: The chickadee? Herring gull? Crow? Black Duck? Starling? House sparrow?
Well, you’d be wrong with any of these. The answer is, the Common Eider, or sea duck. Now, it’s possible to have lived many winters on the Cape and Islands without seeing any eiders, or at most a few. That’s because they don’t come to the local dump like herring gulls or to your backyard feeder like chickadees. Nonetheless, the Common Eider is, by far, our most abundant wintering bird species.
You can prove this to yourself by taking a short boat trip to the shoals off Monomoy Island or Nantucket - not something most of us are likely to do in February. On those shoals are abundant beds of mussels, which form the eider’s primary food source. And it’s here you’ll find the largest wintering rafts of common eiders on the entire Atlantic Coast.
In some years, the estimated count of these flocks has exceeded 500,000 birds, or roughly the population of Boston. Although such large rafts are not usually visible from land, you can commonly see smaller flocks just out beyond the breakers anywhere on the Outer Beach, or in Cape Cod Bay or Nantucket Sound. Last February, for instance, I found a flock of 600 birds in Nauset Harbor.
The eider is the largest of the sea ducks and easy to recognize, though you may need binoculars to see one up close. The male has a pied appearance – white back, black belly, white head, black eye and head cap, a thick goose-like bill, and a nape tinged with green.
Like other sea ducks, eiders are remarkably strong divers and swimmers. They reportedly dive so quickly that they can spot the flash of a gun and be underwater before the bullet reaches them. They dive 50 or 60 feet to pluck mussels, clams, and small crustaceans off the bottom rocks, swallowing their food whole and crushing it with their powerful stomach muscles. They apparently use their wings to swim underwater and have been observed rising from below the surface and into the air while flapping them, as though simply moving from a more dense to a less dense medium.
Despite their local abundance here in the winter, eiders have experienced a mysterious and disturbing die-off in recent years. Beginning in 1998, dead eiders have been found during late summer and early fall along the Cape’s beaches in numbers ranging from a few dozen to as many as 3,000. Until recently the cause of these die-offs remained a mystery, but in 2007 a team of scientists identified the pathogen as a virus. Most of the dead eiders have been found near their feeding grounds at Jeremy Point in Wellfleet Bay. As a result, my hometown now has the dubious distinction of having the virus named after it. Its official name is the Wellfleet Bay Virus, or WFBV. What its origin is, how the virus is transmitted, and what, if anything, can be done to control it, remains unknown.
Still, it’s worth peering out over the winter surf to catch a glimpse of these birds. After all, you’ve got most of the Atlantic Coast population to choose from until late March or early April, when the eiders begin to head back north to their Canadian breeding grounds.
