USFWS to Allow Mute Swan Cull

Recent news stories have told of a decision by the U. S. Fish and Wildlife Service to allow a mute swan cull in Connecticut. The FWS ruling is part of an effort to reduce the number of mute swans along the Atlantic Flyway. The FWS stated, "The need for the action stems from documented scientific evidence of the negative impacts that a growing population of mute swans is having on wetland habitats and native species of fish and wildlife, the threats that mute swans pose to human health and safety, and the damage that they can cause to commercial agricultural crops."

The FWS rationale for mute swan management was set forth in an environmental assessment (references omitted):

Alarmed by recent rapid growth of the population, and aware of the potential and actual detrimental impacts that exotic waterfowl such as the mute swan (Cygnus olor) can have on native plant and animal communities, wildlife professionals have argued the need for a coordinated and cooperative program to reduce mute swan populations to predetermined and manageable levels designed to minimize ecological impacts.

The mute swan is not native to the U.S. It was first introduced to the U.S. in the late 1800's as an ornamental waterfowl to grace the gardens, pools, and ponds of the great estates of the upper classes. The importation of exotic birds, especially those familiar to recent immigrants from the "Old Country," was quite fashionable at the time. Now, one hundred years later and with mute swan populations well established along the Northeast Atlantic Coast and in the Great Lakes Region, the mute swan is one of four naturalized species of birds to be considered invasive, the others being Rock Dove (Columba livia), European Starling (Sturnus vulgaris), and House Sparrow (Passer domesticus).

The mute swan is a frequent subject of children's books, fairy tales, nursery rhymes, literature, music, and dance because of its graceful form and beauty. Thus, it is easy for people to identify with, and form strong bonds of attachment to mute swans. Indeed, a small segment of the public harbors strong emotional, sentimental, and spiritual bonds of affection for mute swans because of their pleasing aesthetic nature. These psychological and social factors do not alter the fact that mute swans are quite destructive of wetland habitats and that their territorial behavior conflicts with other avian species and sometimes with human neighbors.

Mute swans feed almost exclusively on submerged aquatic vegetation, or SAV. SAV is a collective term used to describe a variety of aquatic plants and algae that grow in freshwater and estuarine environments. SAV support incredibly diverse communities of freshwater and marine organisms. SAV beds are the lifeblood of aquatic ecosystems. They provide aquatic nurseries, protective habitats where the young of commercially and recreationally important shellfish and finfish can live in relative security from predators while maturing into adults. Mute swans have been documented feeding on at least 23 species of SAV, including 16 species of pondweeds - sometimes referred to as "grasses" and 7 species of algaes.

In coastal Sweden, [it has been] calculated that 45 mute swans consumed 8,635 kilograms (18,997 pounds) of sea lettuce (Ulva sp.) during a 45-day period, or about 4.3 kilograms (9.4 pounds) per swan per day. In Rhode Island, penned mute swans ate 3.8 kilograms (8.4 pounds, wet-weight) of aquatic vegetation per day. In the Chesapeake Bay, [it was] determined that male mute swans ate 34.6 percent of their body weight per day, while females ate 43.4 percent; applying these figures to average mute swan body weights É, we calculate that males and females consume 3.73 kilograms (8.2 pounds) and 3.65 kilograms (8.0 pounds), respectively, of aquatic vegetation in an average day.

The quantity of SAV eaten by mute swans is only part of the problem. Additional losses occur through the foraging behavior of mute swans. Their consumption of immature seeds, removal of SAV biomass before plant maturation, and uprooting of whole plants may have a very negative effect on the availability of SAV with minimal consumption. Because adult mute swans tend to paddle and rake the substrate to dislodge food for themselves and their cygnets, much vegetation is destroyed and uprooted that is not eaten. Mute swans also use large amounts of vegetation for nest building.

When present in high concentrations, mute swans can over-graze an area, after which they abandon it. The findings of a recent exclosure study in Rhode Island indicated that mute swans can overgraze SAV when water depths are shallow (0.5 meters, or 1.5 feet), often reducing SAV biomass by as much as 92 to 95 percent. In Connecticut, overgrazing of SAV was most severe in the smaller ponds used by breeding pairs, but not evident in larger areas.

Mute swans increase their feeding rate during spring and summer because more food is required prior to feather molt and egg laying, which probably influences the availability of SAV to fall migrant waterfowl. During winter, mute swans probably consume nutrient storage overwintering structures (tubers) which probably has a long-term impact on macropyte availability and species composition. Thus, mute swans can reduce the availability of certain wetland plant species, which can ultimately reduce the carrying capacity of wetlands for native waterfowl.

SAV provides important habitat for a myriad of animal species, and foraging activities of mute swans alter the structure of SAV beds in qualitative or quantitative ways that make them less suitable for the many organisms that depend on this habitat. The varied structure of the SAV beds provide estuarine-spawning fish (e.g., shad, herring, and rockfish) and other marine organisms (e.g., oysters and blue crabs) and their offspring with protection from predators. Any alteration or destruction of the SAV beds - such as can be inflicted by foraging mute swans - would diminish their value for these commercially important species.

Because of their strong territorial nature, mute swans occupy and defend large (up to 6-hectare, or 15-acre) parcels of wetland habitat during nesting, brood rearing, and foraging , and some pairs will vigorously defend nest or brood sites from intrusion by other species of waterfowl. Not only can they attack and displace native waterfowl from breeding and staging areas, they have also been known to kill intruding birds of other species and their young. Mute swans have been reported to cause nest abandonment in Common Terns (Sterna hirundo), Forster's Terns (S. forsteri), Least Terns (S. antillarum) [threatened in Connecticut], and Black Skimmers (Rynchops niger). This aggressive territorial defense by mute swans - an adaptive mechanism to ensure the availability of food resources needed to support their offspring - is clearly detrimental to native birds and wildlife.

The FWS also described earlier efforts to control the mute swan population:

Prior to 2002, there were no mute swan population control measures underway in Connecticut. Although the hunting of mute swans is prohibited by law (Section 26-94 C.G.S.), the Commissioner of Environmental Protection has authority to implement control measures for mute swans pursuant to Section 26- 3 C.G.S. The Bureau of Natural Resources has established a population objective of approximately 190 birds. This number presumes that (1) swans will be excluded from all high-quality habitats (e.g., Federal, State, and private lands) that are specifically managed for biodiversity, including but not limited to wildlife management areas and natural area preserves, and (2) limited numbers of swans will provide viewing opportunities for the public in habitats of low value to native wildlife (e.g., public parks, large lakes, and private ponds).

U.S. Fish and Wildlife Service, Environmental Assessment for the Management of Mute Swans in the Atlantic Flyway, June 2003. See http://migratorybirds.fws.gov/ issues/muteswan/RecordofDecision.htm for links to the complete documentation and references.

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Last updated 18 September, 2004 .