Understanding the Impacts of Invasive Mute Swan Populations in Illinois

Seeing a pair of large, white swans peacefully swimming on a pond or lake is one of the more majestic scenes enjoyed by bird watchers and wildlife enthusiasts. There’s just something about the grace and beauty of these large birds that is a sight to behold. In Illinois, it is not uncommon to see mute swans (a specific species of swan) on both private and public lakes, ponds, and in water features. The mute swan (Cygnus olor) is originally native to Eurasia. First introduced to North America in the late 1800s, and to Illinois in 1971, this large, white swan species was utilized as a picturesque bird for ponds and lakes. Unfortunately, once released, these exotic swans quickly became invasive, with our native ecosystems unprepared and native wildlife ill-equipped to cohabitate with them.

Mute swans can weigh up to 25 pounds and have a wingspan of nearly 7 feet. They are distinguishable from other swan species by their orange bills with a large black knob at the base. Illinois’ native swan species, trumpeter swans (C. buccinator) and tundra swans (C. columbianus), have all-black bills. Mute swans are primarily herbivorous, with a diet consisting of submerged aquatic vegetation. They typically consume between 4 and 8 pounds of vegetation daily, potentially uprooting plants and other vegetation they may not even consume. Submerged aquatic vegetation plays an important role in aquatic ecosystems by providing both food and cover to a variety of vertebrate and invertebrate species. Both the total consumption and foraging style of mute swans can be detrimental to the flora and fauna in these freshwater wetland ecosystems.

Illinois’ largest native swan, the trumpeter swan (C. buccinator), was extirpated from the state by 1900 and its population declined dramatically across the region. Many states in the Great Lakes Region have invested significant resources over the years to re-establish trumpeter swan populations. Unfortunately, mute swans compete directly for food and habitat with the native trumpeter swan along with other waterfowl and colonial waterbird species. In addition to food resources, mute swans are often aggressive towards trumpeter swans and other waterfowl, keeping them from high-quality nesting territories. Mute swan aggression was observed to be highest in the spring during the territory establishment period, which is also a critical time for other waterbirds attempting to establish their own nest sites.

Adult mute swans may also become dangerously aggressive towards people and pets, especially in areas where they have become accustomed to supplemental feeding from humans. In numerous cases, mute swans have threatened and attacked people in parks, backyards and small boats. Although this often involves the swans bluffing to some degree, mute swans are capable of inflicting bruises, sprains and bone fractures; and in 2012, caused one human fatality in Illinois. Aggressive tendencies vary widely between individual swans but are most pronounced in territorial, breeding males.

In 2017, wildlife biologists from the Illinois Department of Natural Resources (IDNR) and USDA Wildlife Services (USDA-WS) initiated efforts to better understand the distribution and density of mute swans in northeast Illinois. To accomplish this, an annual survey was conducted via helicopter in mid-September to estimate the number of mute swans occupying lakes and other bodies of water around the greater Chicago metropolitan area. While these aerial surveys are most likely an undercount, it is a starting point to develop a better understanding of mute swan populations in the region. Since 2017, an average of 111 waterbodies were surveyed each year, with the greatest being 149 in 2022. The average percent of waterbodies occupied by mute swans was 21 percent and the total number of mute swans observed has been around 200 each year (Figure 1).

Because mute swans are nonnative species, they have no federal protections. However, the Illinois Wildlife Code does not differentiate between native and non-native swan species and does extend state protections to cover mute swans. This can present logistical problems for wildlife managers trying to prevent conflicts and combat some of the negative ecosystem impacts of mute swans. IDNR is in the process of developing management strategies for addressing issues with mute swans, both on public and private lands. Properly managing wild mute swan populations can have significant benefits for native wildlife species and their associated habitats, and help prevent conflicts between mute swans and people or pets.

While IDNR and USDA-WS continue to assess how to move forward with management of wild mute swans, spreading awareness and information in Illinois is important to enhance public understanding of some of the associated issues.

Individuals with domestic mute swans have an important responsibility in caring for their birds and preventing some of the conflicts outlined in this article. As we expect and hope for more nesting native trumpeter swans in Illinois in the future, we will need to consider how to manage one of their direct competitors!

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Eric Ness grew up in central Illinois and received a degree in Zoology from SIU Carbondale. He traveled throughout the Midwest working various wildlife technician positions and would go on to receive a Master’s degree in Wildlife Ecology from the University of Delaware. He worked as a biologist in Delaware for several years, returning to his home state of Illinois in 2021.

Anthony Hoffman grew up in northwest Ohio and attended West Virginia University where he received his B.S. in Wildlife and Fisheries Resources. While working various wildlife and natural resources technician positions throughout the Great Lakes Region, he settled in Illinois in 2014. For nearly a decade, his focus has been wildlife damage management.

Ben Williams is the IDNR Division of Wildlife Resources Urban Waterfowl Project Manager.