State and federal biologists have ruled out several causes in the deaths of 850 ducks this winter and were still trying to figure out how they died.
Avian flu, bacterial infections and exposure to heavy metals and toxins have been ruled out as culprits in the only mass duck die-off its kind nationally, said Bruce McCloskey, director of the Colorado Division of Wildlife.
"We've got more answered questions than we have answers," McCloskey said Thursday.
But the worst appears to be over, McCloskey said. Just a few dead ducks have been found in the past few days.
Starting in January, several dead ducks were found at a northeast-Denver wastewater treatment plant. Others were found at other treatment plants and a lake between Boulder and Highlands Ranch.
McCloskey said biologists believe that most of the ducks died from hypothermia after losing their waterproofing and getting wet. One suggestion is that detergents or substances to keep water from foaming might be responsible.
The cold, snowy weather likely contributed to the deaths, McCloskey said. About 50 ducks found along the South Platte River were dry but had starved to death.
Most of the ducks were Northern Shovelers, which have a large, spoon-shaped bill.
Division of Wildlife spokeswoman Jennifer Churchill said the ducks have wintered in Colorado for the past several years.
Some ailing ducks were nursed back to health at a sanctuary and released. About 40 ducks are still being treated.