Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be increasing, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The global population is estimated to number approximately c.598,000-615,000 individuals (Wetlands International 2015). The European population is estimated at 83,400-116,000 pairs, which equates to 167,000-231,000 mature individuals (BirdLife International 2015).
The overall population trend is increasing, although some populations have unknown trends (Wetlands International 2015). The European population is estimated to be increasing (BirdLife International, EBCC 2015).
Behaviour Truly wild populations of this species are migratory (particularly where displaced by cold weather) (del Hoyo et al. 1992, Snow and Perrins 1998) although European and feral populations are essentially sedentary (Johnsgard 1978, del Hoyo et al. 1992, Scott and Rose 1996, Snow and Perrins 1998) or only locally migratory or nomadic (Scott and Rose 1996, Snow and Perrins 1998). The species breeds during the local spring (del Hoyo et al. 1992, Kear 2005a)as isolated pairs in well-defended territories (del Hoyo et al. 1992). After breeding the adults may gather in large concentrations of thousands or more (Johnsgard 1978, Madge and Burn 1988) on selected waters (Madge and Burn 1988) (non-breeders in northern Europe migrating to such gatherings (Snow and Perrins 1998)) between July and August (Scott and Rose 1996) to undergo a flightless moulting period lasting for 6-8 weeks (Kear 2005a). Although not noticeably sociable in many areas during the winter (Johnsgard 1978) the species may flock in groups of several thousands on favoured waters (Johnsgard 1978, Madge and Burn 1988, Scott and Rose 1996). Habitat The species inhabits a variety of lowland freshwater wetlands (del Hoyo et al. 1992) such as shallow lakes (Kear 2005a), ponds (Madge and Burn 1988), lagoons, marshes (del Hoyo et al. 1992), reedbeds (del Hoyo et al. 1992, Snow and Perrins 1998) and slow-flowing rivers (Kear 2005a) (showing a preference for clean, weed-filled streams over larger, polluted rivers) (Johnsgard 1978). It is also common on artificial waterbodies such as reservoirs, gravel-pits, ornamental lakes (del Hoyo et al. 1992), ditches (Snow and Perrins 1998) and canals (Scott and Rose 1996), and will graze on grassland and agricultural land (e.g. arable cereal fields) (Kear 2005a). Moulting congregations of adults and non-breeders (Snow and Perrins 1998) may also utilise brackish or saline habitats (Johnsgard 1978) including brackish marshes (Kear 2005a), estuaries and sheltered coastal sites (del Hoyo et al. 1992) (e.g. brackish lagoons (Kear 2005a) and bays (Madge and Burn 1988)). Diet Its diet consists predominantly of leaves and the vegetative parts of aquatic plants (Johnsgard 1978, del Hoyo et al. 1992) and grasses (del Hoyo et al. 1992) as well as algae (Johnsgard 1978) and grain (del Hoyo et al. 1992), occasionally also taking small amphibians (Johnsgard 1978, del Hoyo et al. 1992) (frogs, toads and tadpoles) (Snow and Perrins 1998) and aquatic invertebrates (e.g. molluscs, insects and worms) (Johnsgard 1978, del Hoyo et al. 1992). Breeding site The nest is a large mound of aquatic vegetation (del Hoyo et al. 1992, Kear 2005a) placed close to or floating on shallow water (del Hoyo et al. 1992, Kear 2005a) or amongst reeds (del Hoyo et al. 1992). Breeding pairs often re-use nesting sites from previous years if the it was successful (Johnsgard 1978). Management information The cyclical removal of adult fish from an artificial waterbody (gravel pit) in the UK resulted in an increase in the growth of submerged aquatic macrophytes and in turn led to an increase in the winter use of the habitat by the species (Giles 1994). The removed fish (dead or alive) were sold to generate funds (Giles 1994). A control of the breeding output of the species (brood reduction) carried out in the Wylye Valley, UK to try to alleviate the species's negative impacts on fisheries (e.g. by overgrazing submergent riverine vegetation) was found to be ineffective as it had an insignificant impact on local population sizes (possibly due to immigration from surrounding areas) (Watola et al. 2003).
The species suffers heavy losses from lead poisoning due to ingested lead fishing weights (del Hoyo et al. 1992, Kelly and Kelly 2004), lead shot (Spray and Milne 1988) and lead contaminated sediments from mining and smelting activities (Day et al. 2003). Heavy losses have also been recorded from local incidences of copper poisoning (Kobayashi et al. 1992). The ingestion of or entanglement in fishing lines and/or hooks can also cause severe injury or mortality (Kelly and Kelly 2004) as can collisions with overhead lines (del Hoyo et al. 1992). The species may be threatened by future oil spills (which can cause death by oil saturation) (Berglund et al. 1963). The species is also susceptible to avian influenza (Melville and Shortridge 2006)(e.g. strain H5N1) (Nagy et al. 2007) so may be threatened by future outbreaks of the virus. Significant mortality can occur with prolonged freezing weather in winter which prevents birds from feeding (Carboneras and Kirwan 2013).
Conservation Actions Underway
EU Birds Directive Annex II. CMS Appendix II. In the U.K. management to decrease competition has shown to be successful (Giles 1994).
Conservation Actions Proposed
Legislation to ban the use of lead weights in fishing and measures to minimise mortality from fishing lines and nets should be introduced. The creation and protection of wetland habitats, particularly from mining waste would also benefit the species. Powerlines should be moved or made more visible.
Text account compilers
Ashpole, J, Butchart, S., Ekstrom, J., Malpas, L.
BirdLife International (2019) Species factsheet: Cygnus olor. Downloaded from http://www.birdlife.org on 17/09/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 17/09/2019.