An outbreak of botulism at Horicon National Wildlife Refuge in Dodge County has killed more than a thousand ducks this summer.
It's the largest die-off at the refuge since the 1970s and is about a tenth of the resident mallard population.
And while some ducks die every year at the refuge because of avian botulism, a biologist says there are many more deaths than usual.
A combination of hot weather and low water levels caused the die-off in the 20,000 acre federal section of the refuge.
The mallards are eating maggots that feed on rotting catfish that are dying because of the low water. The maggots carry a natural bacterium that becomes toxic in high heat and with decaying organic matter.