Denver Parks and Recreation says the birds die off at the park at about this time every year.
What is different about this year is that the lake is being renovated.
In February, workers began to drain some of the water out for an improvement project.
The man-made lake had water quality problems, according to officials.
The project will re-contour the lake-bed once all the water is drained and the bottom is dry.
In the meantime, the lower water levels mean higher concentrations of whatever is in the water, making the birds sick.
"One of the reasons we have the issue of Avian Botulism is because of the number of waterfowl in City Park. Birds, they come into the park, they're fed, they have Kentucky blue grass to eat, they don't migrate like they're supposed to and so you have an increase in the number of birds, and it all contributes to the health risk that we do see with the waterfowl there," says Jill McGranahan with Parks and Recreation.
Avian botulism poses no threat to humans.
"There's absolutely no reason to avoid using City Park, It's perfectly safe," McGranahan said.
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