Nearly 700 birds died or became very ill this summer at Poplar Island, a dredge-spoil island being built in the middle of the Chesapeake Bay that has become key habitat for shorebirds and waterfowl.

More than half the birds were waterfowl that died of botulism, a nerve toxin created by the bacteria clostridium botulinum. . The bacteria are present in the wetland sediments and found in shallow lakes and waterways. It is anaerobic, meaning it can live without air. The first birds die from ingesting the toxin as they feed. Their carcasses attract maggots. The next wave of birds eat the maggots, become infected, and die.

In a separate incident after the waterfowl die-off, biologists discovered hundreds of nesting cormorants had fallen victim to an avian virus called paramyxovirus, which causes muscles to atrophy and makes eating difficult.

At Poplar, the first deaths occurred in late June and affected waterfowl in the island’s upland habitat, said Peter McGowan, wildlife biologist with the U.S. Fish and Wildlife’s field office in Annapolis. These waterfowl were also afflicted with New Duck Disease, another bacterial infection. The sick ducks usually died within 48 hours, first becoming listless and unable to swim and often showing signs of severe injury, such as a twisted neck.

A month later, McGowan said, teams were back on Poplar to investigate the deaths of 263 cormorants.

McGowan said the last time Poplar had a large bird die-off was in 2004. Last summer’s die-offs occurred in pockets of wetlands on the island that have not yet been restored. McGowan said he suspects birds are dying of similar bacteria and viruses in small numbers elsewhere on the Eastern Shore, but that biologists aren’t watching there.

“We’re seeing things at Poplar because we have a lot of eyes out there,” he said. “We are in the marshes and wetlands.”

Poplar had eroded to only a few acres in 1998 when the Army Corps of Engineers and the Port of Baltimore began a $667 million restoration project that has added more than 1,140 acres of dredge material to the island. The new habitat has attracted thousands of birds, waterfowl, turtles and fish. Poplar has been hailed as a success, environmentally and economically, as the Port of Baltimore has become a multi-billion-dollar business.

But at the same time Poplar has become a haven for birds, it has also become a “hot spot” for botulism. That is because of its water levels, said Sallie Welte, director of the Tri-State Bird Rescue. Apparently, engineers are having a hard time regulating the levels so they are deep enough to combat the disease. And weather is a factor; if it doesn’t rain, the water levels remain shallow.

Welte’s organization is trying to stop the spread of the disease by removing infected birds and attempting to revive them; if they survive, she said, they do not return to Poplar.

Welte said her organization responded to smaller die-offs at Poplar over the past four summers, but this summer’s numbers alarmed her.

“It’s very frustrating to have the same problem occur again,” she said. “I don’t know what the solution is.”