Skip to content

Connect with WHOI:

Scientists Investigate Mysterious Duck Die-offs

Scientists Investigate Mysterious Duck Die-offs

Droves of eider ducks have died in five mass mortalities since 2006 on Cape Cod


Andrea Bogomolni was in a skiff near shore when she saw the ducks in October of 2007: “It was surreal,” the biologist remembered. “You could see hundreds of lifeless brown, black, and white lumps on the beach, a foot apart from each other. It was just like they fell out of the sky and dropped dead.”

Seabird ecologist Julie Ellis had already heard about the ducks dying on Cape Cod, but it didn’t prepare her for the sight. Hundreds of common eider ducks were cast ashore, bedraggled and dead, at the wrack line, where the receding tide leaves debris, “lying just one after the other,” she said.

Since 2006, about 2,000 of the sea ducks have died in five mass mortalities on Cape Cod’s beaches. Scientists and conservationists are rallying to find out why.

Ellis is the director of the Seabird Ecological Assessment Network (SEANET), based at the Tufts University Cummings School of Veterinary Medicine, which brings together groups of academic and government researchers, along with citizen scientists, in efforts to identify and mitigate threats to marine birds throughout the Atlantic coast of North America. The mystery of the eider duck die-offs has pulled together Ellis, Bogomolni, a biologist at Woods Hole Oceanographic Institution (WHOI), WHOI biologist Michael Moore, and other researchers at Tufts University, Cape Cod National Seashore, Cape Cod Stranding Network, U.S. Fish and Wildlife Service, and the New England Aquarium. An unusual partner, the hunters’ association Ducks Unlimited, has also joined in the effort to find out what’s killing the eiders.

Collecting specimens and testing suspect causes, the researchers have found and verified one potential cause, but it doesn’t explain all the deaths. There may be multiple causes, ranging from parasites to invasive species, bacteria, viruses, toxic algae, and pollution.

Eiders down

Eiders are northern sea ducks, known for their beauty and exquisitely fine down feathers, which were used for centuries to make comforters known as “eiderdowns.”

Common eiders are the largest ducks in North America, nearly 2 feet long and 4 pounds. They breed in summer on far northern coasts in Alaska, Hudson Bay, Canada, and Maine. They migrate south in autumn and spend winters along coasts south of their summer homes. Many thousands of eiders migrate to spend winters in southern New England, especially on Cape Cod Bay, where both birders and hunters appreciate them.

Eiders have a tough time. Hunted nearly to extinction once, they rebounded, only to see more problems in the last three decades. Common eider populations have declined sharply in some areas (the Pacific and the Arctic) since the 1970s. Some experts believe that Common Eiders are also declining on the Atlantic coast of North America. Hunting, food availability, parasites, the condition of reproductive females, high mortality rates among ducklings, and diseases all may be contributing to the drop.

People have seen and reported mass eider deaths before. On Cape Cod beaches, mortalities have been annual or semi-annual since at least the 1980s, but this year die-offs seemed to happen more often.

Mussels to crabs to worms

In the early spring of 2006, an eider duck die-off occurred in Wellfleet, Mass. Tufts veterinary students discovered the ducks were infected with intestinal parasites known as acanthocephalans (“spiny-headed worms”). Such parasite infections are common in sea ducks, and these worms cause eider mortalities in other regions.

The case seemed simple enough: The evidence pointed to a direct link between seasonal parasites and duck deaths. Cape Cod is a major overwintering site for eiders, whose favorite prey is blue mussels. But populations of the bivalves may have declined in recent years.  “If eiders can’t get mussels,” Ellis said, “they’ll eat snails and crabs—mainly Asian shore crabs and green crabs, because that is what’s available.”

These two crabs, both non-native species that have been introduced to Cape Cod, have increased in numbers. Crabs are the parasitic worm’s intermediate host. “The worm’s juvenile form lives inside crabs, the eider eats the crab, the worm embeds in the eider’s gut, grows and breeds, shedding eggs in the duck’s waste,” Ellis said.

When an eider duck eats a crab, it gets a nasty hitchhiker along with dinner. The parasites can occasionally reach numbers that block their host’s gut so that no other food can be absorbed.

“Lots of eiders in the die-off had guts that were full of these acanthocephalan parasites,” Ellis said. “The ducks were emaciated, starving.”

The trail gets complicated

Another large die-off happened in October 2006, and this time the situation was more complicated. Ellis and Moore went to Wellfleet and Great Island, counted 300 to 400 dead birds, and collected many. Tufts veterinary students and Bogomolni performed necropsies and sent samples for analysis to the National Wildlife Health Center in Madison, Wisc.

“We can’t pin down a cause of this die-off,” Ellis said. Many birds were emaciated, but not all—in fact, some, Bogomolni said, “were robust, but seizing uncontrollably.” Some had the parasites, some did not. The researchers ruled out bacterial infections, bird flu, and West Nile virus. They tentatively concluded that the ducks died of a viral infection, but are still trying to isolate a virus from the samples.

Unexpectedly, yet another mortality occurred in summer 2007. It’s unusual to see eiders in bad shape on the Cape in summer, said Ellis. The dead birds were almost all females.

Ellis went out with Moore again to collect dead and dying ducks. They found them on beaches full of beachgoers.

“It could present a public health issue,” said Ellis. Like many marine mammals and seabirds, these eiders harbor bacteria known to cause infection in humans, said Bogomolni. She is currently studying zoonoses (how disease organisms in animals could spread to humans), in collaboration with Moore, Ellis, WHOI biologist Rebecca Gast, Mark Pokras of Tufts University Veterinary School Wildlife Clinic, and Katie Touhey of the Cape Cod Stranding Network.

“But we still know no cause of death for this event,” Ellis said.“It’s not bird flu, or any particular bacteria.”

Susanna Corona from the New England Aquarium, working with SEANET, collected samples of crabs, water, and fish from Wellfleet for analysis by Jim Haney, a scientist at the University of New Hampshire who studies toxins produced by harmful algae called cyanobacteria. Cyanotoxins were found in nearby Rhode Island waters during the summer of 2007. Other samples are being tested for botulism toxin, the dangerous poison sometimes produced by bacteria in improperly canned food.

An unusual ally

The most recent die-off happened in October 2007. This time adult male ducks died. Males migrate first in the fall, then females, said Ellis—offering a clue that the timing may be a factor in whatever killed the ducks.

Bogomolni and Moore collected 15 dead eiders for SEANET. They euthanized five moribund ducks to send to the National Wildlife Center as unfrozen samples, in case freezing the samples might be destroying some evidence of a cause.

“And again we are not solving it,” said Ellis, “and it’s driving us crazy! Because we’re so annoyed that we can’t identify the cause, we decided to do something unusual”—get in touch with duck hunters.

“No one knows what is a normal state of health for wild birds,” she explained. “Jack Renfrew, from the waterfowl and wetlands conservation organization Ducks Unlimited, said, ‘Well, it’s hunting season, and we shoot eiders anyway, so we could give you some healthy eiders’ ” to examine.

“Some might think that’s not the group to go to, to find out how to keep ducks healthy,” Ellis said, “but people don’t always realize how active hunters are in all kinds of conservation efforts. They really care about the ducks.”

Researchers and hunters went to Great Island; the hunters hunted and contributed eiders, and scoters (another sea duck) for comparison.

“Jack Renfrew said, ‘If they’re eating same thing and only eiders get sick, why is that? If they’re not eating same thing, we can find the difference in their food source’ from looking at gut contents,” Ellis said. “We thought it was a great suggestion.”

More volunteers on the case

The hunted eiders looked in good condition, Ellis said. Sarah Courchesne, a Tufts veterinarian who necropsied the dead eiders, and, curiously, found as many if not more acanthocephalans in the gastrointestinal tracts of the hunted birds, compared with the birds in the die-off. That suggests that the parasites may not be the primary cause of these die-offs.

The researchers will send tissue for analyses, “but testing and culturing samples is very expensive,” Ellis said. “Histology is pricey, but it will tell us more at the tissue level, about the difference between the hunted and sick eiders.”

Ellis is also working with the students, who are monitoring eider numbers via a video camera on Nantucket (put in place and monitored by researchers at the University of Massachusetts, Boston) and looking for the parasitic worms in Asian shore crabs that Ellis collects. And she is bringing the science and the mystery to Quincy and Dorchester teachers and their classes, via the Centers for Ocean Sciences Education Excellence program offered through UMass Boston.

To overcome the difficulties of getting accurate counts of dead eiders on Wellfleet beaches, Ellis recently put out a call for volunteers to aid with SEANET’s efforts to monitor the health of Cape Cod seabirds; she offered volunteer training for interested citizens at the Massachusetts Audubon Society’s Wellfleet Wildlife Sanctuary in January. Among the things volunteers will do: walk Cape Cod’s beaches, looking for dead ducks.

Kate Madin

This research is supported through SEANET, the Bernice Barbour Foundation, and the NOAA Oceans and Human Health Initiative.