Has winter body condition varied with population size in a long-distance migrant, the Bewick’s Swan (Cygnus columbianus bewickii)?

  • Kevin A. Wood
  • Julia L. Newth
  • Geoff M. Hilton
  • Eileen C. Rees
Original Article


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.


Animal 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.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Wildfowl & Wetlands TrustGloucestershireUK

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