Population status of Mute Swans

Investigators:

Dr. Scott Petrie, Long Point Waterfowl

Dr. Charles Francis, Canadian Wildlife Service

Mute Swans, endemic to Eurasia, were introduced to North American city parks, zoos, avicultural collections and estates in the late 1800s and early 1900s.  The intentional releases and accidental escape of these birds resulted in a rapidly expanding population along the northeastern Atlantic Coast of the United States.  The 1999 US Atlantic Flyway mid-summer survey counted 12,541 Mute Swans, most of which occur along coastal wetlands from Massachusetts to Maryland.  By the mid-1900s, Mute Swans were recorded on the lower Great Lakes:  the first breeding record of feral Mute Swans in southern Ontario was in 1958.  Following this, Mute Swans expanded their range throughout much of the lower Great Lakes by the mid-1960s and 1970s and the population has been expanding ever since.  High population growth rates suggest that Mute Swans have found a favorable environment in the lower Great Lakes.  It is climatically similar to their native range in Europe, with low natural predation rates and minimal human interference (they have been legally protected in Canada since 1974).

The accidental and intentional introduction of exotic waterfowl can have negative ecological impacts on native species and their habitats (Weller 1969).  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.  For example, North American Ruddy Ducks (Oxyura jamaicensis) threaten White-headed Ducks (Oxyura leucocephala) in Europe, through hybridization and competition, while Canada Geese (Branta canadensis) introduced into the United Kingdom have caused numerous ecological and economic concerns (Hughes et al. 1999).  Several aspects of their ecology suggest that Mute Swans could be a particularly serious problem.  Mute Swans are extremely aggressive and occupy and defend large parcels (up to 6 hectares) of wetland habitat during nesting, brood rearing and foraging (Birkhead and Perrins 1986, Ciaranca 19990, Ciaranca et al. 1997).  Not only can they attack and displace native waterfowl from breeding and staging habitats (Willey 1968b, Reese 1975, Ciaranca 1990, Ciaranca et al. 1997), they have also been reported to kill adult and juvenile ducks, geese and other wetland dependant birds (Willey 1968b, J. Johnson, personal observation).  Furthermore, Mute Swans have been reported to cause nest abandonment in Least Terns (Sterna albifrons), Black Skimmers (Rynchops niger), Forster’s Terns (Sterna forsteri) and Common Terns (Sterna hirundo) (Ciaranca et al. 1997).

Competition in waterfowl will most likely occur on wintering and/or spring staging areas where food is most limiting (Weller and Batt 1988).  Whereas coastal Great Lakes wetlands are most important as staging habitat for native waterfowl, these habitats are used year round by Mute Swans.  Being primarily herbivorous aquatic foragers, Mute Swans consume daily at least 3-4 kg (wet weight) of submerged aquatic plants, including the leaves, stems, stolons and rhizomes (Willey 1968a, Mathiasson 1973, Owen and Cadbury 1975, Allin 1981, Fenwick 1983).  Because adults also tend to paddle and rake the substrate to dislodge food for themselves and their cygnets (Ciaranca et al. 1997), much vegetation is destroyed and uprooted, that is not eaten (Gillham 1956, Willey 1969b, Chasko 1986).  Mute Swans also utilize large amounts of vegetation for nest building (Gilham 1956).
At high densities, Mute Swans can overgraze an area, causing a substantial decline in the availability of submerged aquatic vegetation, before they move on to a new area (Cobb and Harlan 1980, Allin 1981, Allin et al. 1987).  Mathiasson (1973) calculated that 45 swans consumed 8,635kg of sea lettuce during a 45-day period, and determined that Mute Swans could eliminate som plant species from an ecosystem.

Mute Swans increase their feeding rate during spring and summer because more food is required before feather molt and egg laying (Wilmore 1974) which probably influences the availability of submerged aquatic plants to fall migrant waterfowl.  During winter, Mute Swans probably consume nutrient storage overwintering structures (tubers) which probably has a long-term impact on macrophyte availability and species composition.  For instance, perennial species such as Valisneria americana and Scirpus americanus, overwinter as vegetative buds and the survival of these structures is the main determinant of the next seasons growth (see Kautsky 1990).  On an aggregate percent dry mass basis, Valisneria americana is the most important submerged aquatic food source for ducks at Long Point (Knapton and Petrie 1999).  Therefore, Mute Swans reduce the availability of certain wetland plant species which ultimately reduce the carrying capacity of wetlands for native waterfowl.  However, it is unknown which submerged plant species (or which parts of those plants) are the primary forage of Mute Swans on the the lower Great Lakes so it is difficult to predict the possible impact of this exotic species on native waterfowl or their habitats.

As Mute Swan numbers have been expanding rapidly on the lower Great Lakes over the past 30 years, it was deemed critical to initially document the rate of spread of this species and to predict how large the southern Ontario population may eventually become. We used three independent data sources to
estimate the rate of increase of Mute Swans on the lower Great Lakes:  aerial surveys in spring and fall at Long Point, Lake Erie 1971-2000; mid-winter inventory of the north shore of Lake Ontario, 1980-2000; and Christmas Bird Counts on both the US and Canadian shores of the lakes, 1980-2000.  The average estimated population growth rate varied from 10% per year to 21% per year depending on the data set.  The most conservative growth rate estimate of 10% per year indicates a doubling of the Mute Swan population every 7-8 years.

 

We have also initiated studies to determine what foods Mute Swans are utilizing throughout the year at Long Point and Lake St. Clair, ultimately so that we can predict what impact they are having on native waterfowl through exploitation and competition.  Nutrient reserves dynamics of Mute Swans is also being studied.

Literature Cited

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Allin, C.G., G.G. Chasko, and T.P. Husband. 1987. Mute Swans in the
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Birkhead, M. E., and C. M. Perrins. 1986. The Mute Swan. Croom Helm, London.

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Mathiasson, S. 1973. Distribution and behaviour of non-breeding Mute Swans of the Swedish west coast. Vittrery 8:400-452.

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