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color photo of several Mute Swans

Invasive Species in the Chesapeake Watershed

Cygnus olor

What is Cygnus olor?


Included in the family Anatidae with ducks, geese and swans, mute swans are the largest bird in the Chesapeake Bay. Originating from Eurasia, mute swans were transported to northern Europe in the Middle Ages and, subsequently, to North America and have been favored among captive owners and breeders of waterfowl for their beauty and grace. Adult males are larger than females. Average mass for adult males is 10.2 kg and for adult females is 8.4 kg. Average length of males and females is 1.27 to 1.52 m. Adults can have a wing span of about 1.8 to 2.4 m. Adult birds are white and have orange bills with a characteristic black, basal knob and a black terminal nail, whereas the tundra swan, native to North America, has a simple, black bill. Legs and feet of adults can range in color from black to grayish pink. Mute swan cygnets are grayish brown or white, with slate gray legs and feet or pinkish/tan feet, respectively. Cygnets lack the basal knob. White morph cygnets have tan bills, while gray morph cygnets have slate bills.

Mute swans utilize a variety of aquatic habitats, including ponds and lagoons and fresh to salt water marshes. In the warmer months, mute swans spend most of their time in shallow water. As shallow water freezes, the birds move to deeper water, but will utilize deeper water throughout the year.

Population Ecology

Atlantic flyway. The mean annual rate of population growth for mute swans in Maryland was 36% from 1962 to 1979. From 1986 to 1999, the mute swan numbers in Maryland increased from 264 to 3,955, an increase of 1,389%. The 1999 estimate of total mute swans in the Atlantic flyway was 12,600 birds. The growth rate for mute swans in the Chesapeake Bay since 1986 has been 1,271%. A nest survey in the Patuxent River in 2000 revealed five nests; a survey conducted in 2001 revealed 40 nests. Population modeling of Maryland's mute swan population indicates that it could include over 20,000 birds by 2010 if growth is unchecked (Harvey 2000). From 1989 to 1999, according to the Atlantic Flyway Survey, Massachusettes' mute swan population grew 68%, Rhode Island's 79%, New Jersey's 159%, Pennsylvania's 78% and Virginia's 713%.

Mute swans are year-round residents in the Chesapeake Bay and are not true migrants in any part of their range in North America. While occurring throughout the Bay, they are most concentrated from Rock Hall in Kent County south to Hoopers Island in Dorchester County. Until they are old enough to nest, and during the winter months, mute swans spend most of their time in large flocks composed of juvenile, sub-adult swans. Flocks of 600 to 1,000 birds have been recorded in the Chesapeake Bay. Breeding pairs remain on territories most of the year.

Mortality after mute swans reach breeding age is low. Causes of mortality can include disease, severe winter weather, lead toxicosis, collision with high tension wires or other man-made structures and incidental shooting in Virginia, Pennsylvania and Delaware where they are unprotected by state law, . Once they reach breeding age, about 85% survive from one breeding season to the next. Average life expectancy is 11 years and the maximum is 21 years (Ciaranca et al. 1997).

Feeding Habits

Submerged aquatic vegetation (SAV) is the preferred diet of mute swans throughout the world, though they will also eat grain crops. In one Chesapeake Bay study, widgeon grass (Ruppia maritime) constituted 66 and 78% of the food eaten at Eastern Bay and Smith Island, respectively, whereas eel grass (Zostera marina) formed 2% and 32%, respectively, for these areas. Other SAV and invertebrates amounted to only 1% (M. Perry, USGS, Laurel, Maryland unpubl. data). Other SAV important to mute swan diet in the Chesapeake Bay include sago pondweed (Potamogeton pectinatus), clasping-leaved pondweed (P. perfoliatus), horned pondweed (Zannichella palustris) and Myriophyllum spicatum. Adult mute swans consume 1.8 to 3.6 kg of plant material each day (Fenwick 1983) and can reach SAV in 1.07 m of water (Owen and Cadbury 1975). They have been observed pulling plants up by the roots or rhizomes or paddling vigorously to dislodge whole plants to consume or to make available for cygnets (Owen and Kear 1972, Birkhead and Perrins 1986).


Mute swans nest when they reach about three years of age; pairs generally remain together until one member dies, or the remaining member of a pair chooses, or does not choose, another mate. Nesting begins in March or early April and pairs often use the same nest sites over multiple years. Mute swans nest very close to the water on small islands, isolated shorelines or in shallow marshes. The nest is made from rushes and coarse emergent grasses and ranges from 4 to 6 feet in diameter and about 1.5 feet above the high-tide line. The female, or pen, does most of the nest building and is the principal incubator of the eggs. Unlike other waterfowl in the Northern Hemisphere, mute swan males have been observed incubating in the absence of a female (Witherby et al. 1952). Clutch size in the Chesapeake Bay ranges from 4 to 10 eggs, with a mean of 6.2 (Reese 1996). Incubation continues for about 35 days after the first egg is laid, between mid-May and mid-June. Mute swans generally nest once a year, though if a nest is disturbed early in the nesting season and eggs are lost, a pair may attempt to nest a second time. Mute swan pairs, especially males, can be aggressive to other waterfowl, humans and pets that venture into their nesting territory, which can include up to 13 acres. Aggressive defense of territories begins in late February. In rare instances, mute swans will nest in colonies (Maryland DNR files).

Cygnets are precocious. They begin swimming within a day or two of hatching and are fully grown in less than six months. They are independent at 125 to 132 days. In the Chesapeake Bay, 49% of eggs laid survive to hatching and about 83% of hatching cygnets are able to fledge. Cygnets are ready to fly in about four to five months, and may then leave their parents territory. Female mute swans begin to molt in mid-July, while males delay their molt until their female partners regain flight. Most mute swan families break up in the fall, when young birds are forced out by their parents.

Ecological Concerns

Of primary concern to Chesapeake Bay ecologists is the rate of mute swan population growth in the Chesapeake Bay, its presence year-round and its preference for feeding on SAV. Certain SAV species, such as wild celery (Vallisneria americana), are especially vulnerable because their reproduction and growth are timed to avoid the heavy grazing of migratory waterfowl. Wild celery requires its reproduction process to be protected from grazing while its seeds are maturing. If consumed before seeds are mature, it will not reproduce and will waste living energy in this process. A large, resident mute swan population feeding on SAV all year could jeopardize the ability of SAV to recover from winter waterfowl grazing and make it less available for waterfowl the following winter. Declines in SAV abundance appear to correlate with declines in local black duck (Anas rubripes) abundance (Krementz 1991). Population trends suggest that habitat degradation in Chesapeake Bay, especially loss of SAV, may be the principal cause of the decline of the Bay's canvasback (Aythya valisineria) population (Haramis 1991). Furthermore, the loss of SAV over the past several decades has prompted the near abandonment of Bay waters by redheads (Aythya americana), leaving only a remnant population today (Haramis 1991).

In closed waterways in Europe, mute swans have been documented as removing entire species of SAV (Gillham 1956, Jennings et al. 1961, Mathaisson 1973, Chairman 1977, Neirheus and Van Ireland 1978, Scott and Birkhead 1983). In a recent Rhode Island exclosure study, for example, findings indicated that mute swans overgraze SAV when water is shallow (0.5 m), reducing SAV biomass by 92 to 95% (Allin and Husband 2000). The Chesapeake 2000 Agreement includes a commitment to restoring 114,000 acres of SAV; however, restoration efforts, particularly in the mid-Bay where SAV decline is most severe, are frequently obstructed by feeding mute swans.

Mute swans compete with native, wintering tundra swans (Cygnus columbianus) for shelter and food to a lesser extent than shelter. Tundra swans lose mass during the winter and depart from the wintering grounds at their lowest mass (Bortner 1985, Limpert et al. 1987). Harassment by mute swans may cause tundra swans to lose mass more rapidly, which could affect subsequent reproduction. Inter-specific aggression during nesting season has been documented in the Chesapeake Bay between Canada geese and mallard and black ducks.

In the early 1990s, a molting flock of between 600 to 1,000 mute swans utilized a beach area, Barren Island (off of Blackwater National Wildlife Refuge), as a loafing site. This same site was the last remaining nesting site for black skimmers (Rynchops niger) and the last natural nesting site for least terns (Sterna antilarum) in Maryland. The mute swan activity crushed eggs and young of birds nesting in the beach colony, which led to the black skimmers and least terns abandoning this area for three nesting seasons (Maryland DNR files).

Human/Economic Interests

Purple loosestrife, hybrids and cultivars are regulated as noxious weeds in Virginia (§3.1-296.11 et seq.) and Pennsylvania (3 P.S. 255.1 et seq.), but are not listed in Maryland (Dick Bean pers. comm.). The Virginia law declares it illegal to move, transport, deliver, ship or offer for shipment into the state. The Pennsylvania law prohibits sale, transport, planting and propagation. Although it is legal to sell L. salicaria in Maryland, individual nurseries have voluntarily discontinued its sale as a potted plant (Dick Bean pers. comm.).


Complaints about mute swans from citizens vary. Mute swans consume and disturb SAV beds in impoundments or sheltered coves that provide crabbing and fishing opportunities; and aggressive pairs can prevent the use of shoreline or adjacent water for recreation. In large concentrations, mute swans and other waterfowl can contribute to water quality problems by defecating in the water. On Long Island, New York, elevated counts of coliform bacteria have been detected where mute swans congregate. Public health authorities are concerned about the impact of nutrient loading where waterfowl congregate because coliform counts are widely used to determine whether waters may be used for drinking, swimming or shell fishing. Nutrient loading can also cause dangerous algal blooms, especially in inland ponds where rooted SAV has been removed by mute swans (NYDEC 1993).

Regulatory Status

In December 2001, the U.S. Court of Appeals ruled that mute swans are covered under the Migratory Bird Treaty Act (MBTA), which provides the U.S. Fish and Wildlife Service (USFWS) with authority over any activity that directly impacts the birds, their eggs or nests. Prior to this ruling, the USFWS did not consider the mute swan covered under the MBTA, and regultory authority was designated to the states. Now that the USFWS is charged with the authority for managing mute swans, the Maryland Department of Natural Resources, Virginia Game and Inland Fish Commission, the Pennsylvania Game Commission, and the Delaware Department of Natural Resources and Environmental Conservation are responsible for carrying out and or modifying their state regulations under USFWS jurisdiction. The USFWS is currently considering a regulatory mechanism to delegate federal authority over mute swans back to the states. In the interim, states are applying for federal permits to conduct research and population control programs.

Before the December 2001 ruling, mute swans in Maryland were included in the statutory definintion of "wetland game bird," which gave the Maryland Department of Natural Resources jurisdiction over their management. Mute swans were not protected in Virginia, Delaware or Pennsylvania. In 1997, the Atlantic Flyway Council issued a mute swan policy encouraging state wildlife agencies and other resource management agencies to control mute swans in the Atlantic flyway (AFC 2000). In 1996, the USFWS directed all National Wildlife Refuges to control mute swans within their boundaries (USFWS Internal Memo, May 24, 1996).

Management Efforts Overview

State wildlife agencies have attempted various population control measures, including egg addling and relocation or killing of adult birds. Most recently, six states in the Atlantic flyway (Delaware, Maryland, New York, Rhode Island, Virginia, and Vermont) have attempted to control mute swan populations through both passive actions (encouraging landowner/manager control) to aggressive actions (state employees actively removing mute swans from state lands and waters). Vermont, in addition to establishing a policy prohibiting the establishment of wild mute swan populations in the state, has regulated their treatment in captivity: birds are to be pinioned, their sale or importation prohibited, and the eggs addled.

Monitoring of mute swan populations in Atlantic flyway states is conducted by aerial surveys every three years, in mid-summer, when native swans and other migratory birds are not present in the Chesapeake Bay.

Management Efforts in the Chesapeake Bay Watershed


Prior to the U.S. Court of Appeals ruling, Delaware officially treated mute swans as a deleterious species and birds were systematically removed from all public lands.


Maryland is developing a statewide mute swan management plan, including research projects to examine the potential impacts of mute swans on declining populations of wintering tundra swans and on SAV. The state has obtained federal permits for intensive egg addling in 2002, and is developing strict regulations for their sale, importation, breeding and captive management. It is also considering public forums to educate citizens about mute swans and their impacts and to learn more about public perception. Maryland has permited the removal of several hundred swans by game breeders for shipment to Asia. In addition, as part of its mute swan plan, the state has identified sensitive Bay areas to target for exclusion of mute swans, including SAV restoration sites, areas where rare SAV grows naturally and nesting sites for rare birds. Maryland is also considering annual surveys of mute swan population growth and is testing the use of male sterilization in preventing the growth of the population.


Prior to the U.S. Court of Appeals ruling, mute swans were unprotected in Pennsylvania: they could be taken without permit at any time of the year.


Prior to the U.S. Court of Appeal ruling, Virginia permitted the capture and relocation of same sex pairs to inland waters. As an unprotected species, mute swans were open to hunting at any time of the year by hunters or landowners who could demonstrate that the swans presented a conflict or threat. A small number of mute swans were also taken incidentally during limited tundra swan hunting seasons held in Virginia. The Virginia Department of Game and Inland Fish conducted limited egg addling and removal of adult birds.


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