Mute Swans in Maryland: A Statewide Management Plan
Maryland Department of Natural Resources
Wildlife and Heritage Service
April 14, 2003

ECOLOGICAL IMPACTS AND CONFLICTS  

Impacts to Public Safety and Use of Private Property 

Despite their aesthetic appeal, mute swans are a problem for some people. Some birds threaten or directly attack people who get too close to their nest or young. The aggressive behavior exhibited by these large birds can pose a safety risk, especially to small children and persons swimming or in small watercraft. Although the potential for injury is low, many people who experience this display of aggressive behavior are fearful of it. This behavior prevents some shoreline landowners from using their shoreline property and adjacent waters during the nesting and brood-rearing season.

Grazing Impacts Upon Submerged Aquatic Vegetation

Unlike the native tundra swans that only spend winter months in the Bay, the nonnative mute swan inhabits the Bay year-round. Mute swans feed solely on SAV. While foraging, each bird consumes an average of about 8 pounds of SAV each day, including leaves, stems, roots, stolons, and rhizomes. Wintering tundra swans also feed on SAV but also consume clams and waste grain and green grain crops in agricultural fields. Mute swans, on the other hand, feed exclusively in shallow wetlands in Maryland where they consume large amounts of SAV. They also utilize large amounts of emergent vegetation for nest building. Adult mute swans tend to paddle and rake the substrate to dislodge SAV and invertebrates for them and their cygnets; thus, more SAV is destroyed and uprooted than is eaten. At high densities, mute swans can overgraze an area, causing a substantial decline in SAV at the local level.

This consumption of SAV has raised serious concerns among shoreline property owners and resource managers. SAV is critical to the health and well being of a myriad of Bay organisms. Not only does SAV protect water quality and prevent erosion, it also provides food and shelter for fish, shellfish, invertebrates, and waterfowl. For example, research has shown that the density of juvenile blue crabs is 30 times greater in SAV beds than in unvegetated areas of the Bay.

The abundance and distribution of SAV has been greatly reduced during the last 30 years. The decline of SAV has been attributed primarily to elevated levels of nutrients and suspended sediments. However, the grazing of SAV by mute swans places additional pressure on this already stressed and vital resource. Grazing of SAV by mute swans reduces the capacity of the remaining SAV beds in the Bay to support wintering waterfowl and other fish and wildlife populations. Food habit studies show that widgeon grass and eelgrass are the most important foods of mute swans in winter and spring. These SAV species are also important foods for many other wintering waterfowl species.

Although data on the reduction of SAV by mute swans is limited, there is sufficient information to conclude that these birds are having a deleterious impact on SAV in the Bay. Bay scientists and shoreline property owners report concentrations of foraging swans severely impacting SAV beds. Citizen tributary organizations have had SAV and emergent transplantings damaged by mute swans, thwarting efforts to improve water quality. The cost of replanting one 0.06 ha restoration site damaged by mute swans in the South River exceeded $4,000.Today, physical barriers protect nearly all transplant sites from mute swans, at significant additional cost.

Mute swan grazing on SAV has been observed by research scientists, including feeding on reproductive shoots before they mature. Swan foraging on SAV during the spring and summer growing season has been shown to reduce plant survival and reproduction, reducing SAV abundance in subsequent years. Over time, areas with high densities of mute swans exhibit a decrease in plant diversity and abundance, sometimes becoming devoid of SAV.

The presence of a large mute swan population in the Bay is in conflict with public policies aimed at restoring the Chesapeake Bay. A simple mathematical extrapolation of SAV consumption by mute swans suggests that 4,000 mute swans may consume up to 12 million pounds of SAV annually, representing about 12% of the SAV biomass in the Bay. This level of impact is greatest on the mid-Eastern Shore where high numbers of mute swans concentrate and acreage of SAV is small. This level of grazing, especially during spring and fall SAV growth and reproductive periods and in SAV restoration plantings, is an impediment to achieving the objectives identified in the Vital Habitat Protection and Restoration Section of the Chesapeake 2000 Agreement (Appendix B), in particular the goal to “Preserve, Protect and Restore those habitats and natural areas vital to the survival and diversity of the living resources of the Bay and its tributaries.”

Impacts to Property and Agricultural Resources

Few instances of property damage by mute swan have been reported. Currently, there is no evidence to suggest that mute swans are causing any impact to agriculture in Maryland. Elsewhere in the U.S., mute swans have caused economic losses to agricultural crops. In New Jersey, mute swans have caused several thousands dollars of damage to commercial cranberry crops. In Washington State, British Columbia, and in Europe, mute swans feed in agricultural fields. They cause damage to small grain crops (i.e., winter wheat and canola) and pastures in Europe.

Direct Impacts to Native Wildlife

The accidental and intentional introduction of exotic waterfowl has negative ecological impacts on native species. Adverse effects are particularly likely if the introduced species is aggressive, competes with other waterfowl for food or habitat and/or hybridizes with native species. The aggressive behavior exhibited by some mute swans toward humans is commonly directed toward other waterfowl. Observations in Maryland and findings reported in scientific literature support the fact that territorial mute swans can be very aggressive towards other waterfowl, displacing native species from their breeding and foraging habitats.

Mute swans occupy and defend relatively large territories of wetland habitat during nesting, brood rearing and foraging, and thus compete with native birds for habitat. Not only do they displace native waterfowl from breeding and staging habitats, they have been reported to attack, injure, or kill other wetland birds. This is especially true of male swans defending either their nesting territories or cygnets.

The most serious instance of conflict between native wildlife and mute swans occurred in the early 1990’s, when a large flock of mute swans (600-1,000 swans) caused the abandonment of nesting sites for state-threatened colonial nesting birds at Tar Bay in Dorchester County. These colonial nesting birds nested on oyster shell bars and beaches that were used by swans as loafing sites. Tar Bay was the only area in the Maryland portion of the Bay where black skimmers and least terns nested on natural sites.

Maryland has the largest population of mute swans in the Atlantic flyway. There is growing concern among wildlife managers that the increase in mute swans in Maryland is contributing to factors that have suppressed population growth among tundra swans that winter in Maryland. Tundra swans nest in Alaska and Canada and migrate to Chesapeake Bay to spend the winter. While tundra swans wintering along the east coast (e.g., adjacent states of Pennsylvania, Virginia and North Carolina) have increased during the past two decades, tundra swans wintering in Maryland have declined about 40% during the past 25 years.

Mute swan pairs have been observed exhibiting aggression toward wintering tundra swans in Maryland, driving them from foraging areas and protected coves used for winter shelter. Food habit studies show that tundra swans and mute swans do compete for limited SAV food resources, but tundra swans feed on invertebrates and agricultural foods to a greater extent. The extent to which aggressive behavior and competition from mute swans is related to the inability of the state’s wintering tundra swan population to increase is unknown.

Mute swans consume large amounts of SAV that might otherwise be available to native waterfowl. This competition for space and food imposed by mute swans reduces the carrying capacity of breeding, staging, and wintering habitats for native species of migratory waterfowl in Chesapeake Bay where mute swans are established. Numbers of several waterfowl species (e.g., redhead, canvasback, American widgeon, black ducks, and Atlantic brant) dependent upon SAV have declined in the Bay. The declines in these wintering waterfowl populations in the Bay are attributed to the reduced abundance of SAV. Except for black ducks, continental populations of these species are quite healthy, at or above North American Waterfowl Management Plan objectives.