Has winter body condition varied with population size in a long-distance migrant, the Bewick’s Swan (Cygnus columbianus bewickii)?
Assessments of body condition can provide useful information on changes in the state of individuals within a population, which may in turn help to inform conservation efforts. For example, decreases in body condition over time can indicate reduced food resources. Mass and skull length measures recorded for 195 adult and 467 first winter (cygnets) Bewick’s Swans (Cygnus columbianus bewickii) at wintering sites in the UK between winters 1966/1967 and 2017/2018 therefore were analysed to determine whether a ca. 40% decline in numbers in the Northwest European Bewick’s Swan population between 1995 and 2010 corresponded with poorer body condition from the mid-1990s onwards. Parents and siblings were known for all individuals, allowing us to account for shared genetic factors and rearing environment in our analysis. We used linear mixed-effects models and an information-theoretic approach to test different models of temporal variation in scaled body mass index (SBMI). Within our study population, although SBMI values varied both within and between years, we found no evidence of any directional trends in body condition. Of our competing time models of swan SBMI, a model in which age-specific body condition was constant over time received the greatest support in the data. Body condition was greater for adults than cygnets, but did not vary between sexes or wintering sites. Our findings suggest no connection between the recent declines in population size and body condition. Population decline is therefore unlikely to be caused by inadequate food supplies.
KeywordsAnimal biometrics Energy reserves Food resources Long-term ecological studies Species conservation Waterbirds Waterfowl demography
This study would not have been possible without all of the WWT staff and volunteers who helped to capture and measure the Bewick’s Swans. We are grateful to all those who contributed to the co-ordinated international censuses of winter Bewick’s Swan numbers. We thank Robin Jones for assistance with data access, and Carl Mitchell, Dafila Scott, John Arbon and Linda Butler for information on site use by Bewick’s Swans. Thanks also to Christian Gortázar, an associate editor and anonymous reviewer for their helpful comments on an earlier version of this study. This study was funded by the Peter Scott Trust for Education and Research in Conservation, Peter Smith Charitable Trust for Nature, Olive Herbert Charitable Trust, D’Oyly Carte Charitable Trust, N. Smith Charitable Settlement, Robert Kiln Charitable Trust, the estate of the late Professor Geoffrey Matthews OBE and all who supported WWT’s ‘Hope for Swans’ appeal.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Albertsen JO, Abe Y, Kashikawa S, Ookawara A, Tamada K (2002) Age and sex differences in biometrics data recorded for whooper swans wintering in Japan. Waterbirds 25:334–339Google Scholar
- Barton K (2012) MuMIn: model selection and model averaging based on information criteria. R package version 1.13.4Google Scholar
- BirdLife International (2015) European red list of birds. Office for Official Publications of the European Communities, LuxembourgGoogle Scholar
- Black JM, Rees EC (1984) The structure and behaviour of the whooper swan population wintering at Caerlaverock, Dumfries and Galloway, Scotland: an introductory study. Wildfowl 35:21–36Google Scholar
- Bowler JM (1994) The condition of Bewick's swans Cygnus columbianus bewickii in winter as assessed by their abdominal profiles. Ardea 82:241–241Google Scholar
- Dirksen S, Beekman JH, Slagboom TH (1991) Bewick’s swans Cygnus columbianus bewickii in the Netherlands: numbers, distribution and food choice during the wintering season. Wildfowl (Supplement No. 1):228–237Google Scholar
- Evans ME (1982) Movements of Bewick’s swans, Cygnus columbianus bewickii marked at Slimbridge, England from 1960 to 1979. Ardea 70:59–75Google Scholar
- Evans ME, Kear J (1978) Weights and measurements of Bewick’s swans during winter. Wildfowl 29:118–122Google Scholar
- Griffin LR, Rees EC, Hughes B (2016) Satellite-tracking of Bewick’s swan migration in relation to offshore and onshore wind farm sites: final report to the Department of Energy and Climate Change. Wildfowl & Wetlands Trust, SlimbridgeGoogle Scholar
- Limpert RJ, Allan HA Jr, Sladen WJ (1987) Weights and measurements of wintering tundra swans. Wildfowl 38:108–113Google Scholar
- Luigujõe L, Kuresoo A, Keskpaik J, Ader A, Leito A (1996) Migration and staging of the Bewick’s swan (Cygnus columbianus) in Estonia. Gibier Faune Sauvage 13:451–461Google Scholar
- Meier-Peithmann W (2011) Dokumentation des nahrungsokologischen Wandels bei Sing-, Zwerg- und Hockerschwan Cygnus cygnus, C. bewickii, C. olor von 1965 bis 2010 in den Dannenberger Elbbogen. Vogelwelt 132:57–79 [In German with English summary]Google Scholar
- Nagy S, Petkov N, Rees EC, Solokha A, Hilton G, Beekman J, Nolet B (2012) International Single Species Action Plan for the Northwest European Population of Bewick’s Swan (Cygnus columbianus bewickii). AEWA Technical Series No 44, BonnGoogle Scholar
- Newton I (2013) Bird populations. Harper Collins, LondonGoogle Scholar
- Rees EC (2006) Bewick’s swan. T&AD Poyser, LondonGoogle Scholar
- Rees EC, Bacon PJ (1996) Migratory tradition in Bewick’s swans (Cygnus columbianus bewickii). Gibier Faune Sauvage 13:407–420Google Scholar
- Rees EC, Beekman JH (2010) Northwest European Bewick’s swans: a population in decline. Br Birds 103:640–650Google Scholar
- Rees EC, Bowler J (1996) Fifty years of swan research and conservation by the Wildfowl & Wetlands Trust. Wildfowl 47:248–263Google Scholar
- Scott P (1966) The Bewick’s swans at Slimbridge. Wildfowl 17:20–26Google Scholar
- Worden J, Cranswick PA, Crowe O, McElwaine G, Rees EC (2006) Numbers and distribution of Bewick’s swan Cygnus columbianus bewickii wintering in Britain and Ireland: results of international censuses, January 1995, 2000 and 2005. Wildfowl 56:3–22Google Scholar