Thousands of Common Eider carcasses have been found floating in the cold water off Cape Cod or rotting on beaches. Many more of the birds have been found emaciated, exhausted, and suffering, often near death. In recent years, a mysterious virus has taken a mounting toll on the region’s common eiders, a normally bulky sea duck considered a barometer for the health of Cape Cod Bay.
Chris Dwyer, a biologist with the Fish & Wildlife Service, said about 6,000 eiders have been found dead over the past decade, most near their feeding grounds by Jeremy Point on Wellfleet Bay. The deaths have occurred mainly in the fall, a few hundred at a time. But there have been spikes, such is in 2007, when officials found some 3,000 dead along Jeremy Point.
Dwyer said the losses have been steady but don’t appear to be increasing. Between 250,000 and 300,000 common eiders migrate every year along a corridor from the coast of Labrador in Canada, where they breed, to as far south as Long Island Sound, where they feed on mollusks and other crustaceans. The farthest south they breed is the Boston Harbor Islands.
“If we ignore this, we worry that it could take off quickly,” Dwyer said. “A healthy eider population indicates a clean environment. We worry about the ripple effects if this gets worse.”
Wildlife officials first began noticing premature deaths and sickened eiders in Wellfleet Bay in 1998. Those found alive with the virus tend to be stricken with lethargy, diarrhea, and troubled breathing, as the disease attacks their liver and gallbladder.
Scientists believe the virus may be transmitted by ticks or other insects, such as mites or mosquitoes. Other birds that have been found with the virus, including ring-billed gulls and white-winged scoters, have yet to show signs of illness.
But Jennifer Ballard, a wildlife disease diagnostician studying the disease at the University of Georgia in Athens, said it is possible the virus had existed in the birds for years but is only being detected now. She said the virus is genetically related to a group of viruses found in other migratory sea birds that nest in similarly sized colonies.
Identifying the source of the virus, she said, is important because it will help wildlife officials better manage the eider population and maintain balance in the ecosystem. The federal government allows hunters to bag about 30,000 eiders a year..
Over the next few days, Dwyer’s team caught 33 more eiders, implanting 19 transmitters in all. They didn’t find any ticks, but Dwyer said they collected soil samples from the nests and enough blood and feathers that he hoped they would yield insight into the cause of the lethal virus.