An outbreak of avian botulism is affecting wildlife along Chicago’s lakefront this summer, raising concerns among biologists and conservationists about the health of local bird populations. The phenomenon has been observed across a stretch of shoreline from Rainbow Beach on the city’s South Side to the Indiana border, with mallard ducks, herring gulls, and ring-billed gulls among the most commonly impacted species.

Officials from the Forest Preserves of Cook County, working alongside state authorities, have been conducting pathology examinations and collecting bird carcasses to monitor the outbreak. “Almost every summer, we have cases,” said Annette Prince, director of the Chicago Bird Collision Monitors. The volunteer group has received between six and a dozen reports of suspected cases, including locations near Ohio Street Beach and the Museum Campus, illustrating the connectivity of affected areas along the lakefront.

Avian botulism is caused by a toxin produced by the bacterium Clostridium botulinum, which thrives in warm, low-oxygen environments such as stagnant or warming surface waters. Since early August, Lake Michigan’s surface temperatures have ranged from the upper 60s to lower 70s Fahrenheit, conditions that foster the bacterium’s growth. Prolonged drought and reduced water levels exacerbate the problem by exposing decaying vegetation along the lakeshore, which serves as a breeding ground for the toxin.

Birds become infected either by ingesting the toxin directly through contaminated plants or indirectly by eating infected fish and invertebrates. The disease is not contagious from bird to bird but spreads through a cyclical pattern involving maggots that develop in the carcasses of infected birds and then infect other birds that consume them. This cycle can rapidly increase mortality rates, sometimes escalating from a few deaths to hundreds or more.

Symptoms of avian botulism include progressive paralysis manifested as drooping wings, weakness in the legs, and limp necks, often leading to death by drowning or respiratory failure. The type of botulism typically seen in the Great Lakes region—type E—has been recognized since the 1960s, coinciding with increasing nutrient runoff that creates low-oxygen zones ideal for toxin production. Research from the U.S. Geological Survey and the University of Wisconsin-Madison suggests that warming lake temperatures and algal blooms, both linked to climate change, are contributing to a rise in avian botulism outbreaks.

Invasive species may also be playing a role. The round goby, a bottom-feeding fish introduced to the Great Lakes in 1990, can carry the botulism toxin, which is then passed to birds that feed on them. Additionally, invasive zebra and quagga mussels increase water clarity, promoting algal growth that eventually depletes oxygen as the algae decay.

The outbreak has drawn public attention, with local residents reporting dead and dying birds to authorities. In response, state and county officials are coordinating cleanup efforts and monitoring the situation closely. Chris Anchor, a senior wildlife biologist with the Forest Preserves of Cook County, urged the public to report clusters of bird deaths, noting that while individual deaths are common and expected, multiple fatalities in a concentrated area often indicate an outbreak requiring investigation.

Lake Michigan’s water levels this year have fallen below their long-term averages for the first time in a decade, a factor that, combined with shifting precipitation patterns—characterized by intense rainfall events separated by extended dry spells—may be contributing to the recurrence and severity of avian botulism outbreaks along Chicago’s lakefront.