GLEN ARBOR, Mich. (Michigan News Source) – Last weekend, visitors to Sleeping Bear Dunes National Lakeshore encountered a grisly site.

An estimated 300 dead birds were found over the weekend along the park’s beaches. Officials said the birds likely died from an outbreak of type-E botulism, also known as avian botulism.

Most of the dead birds were long-tailed ducks and loons, among others.

Water temperatures, lake levels, and invasive species are considered some of the key factors in the spread of the disease.