The condition of Bewick’s Swans Cygnus columbianus bewickii in winter as assessed by their abdominal profiles.

An Abdominal Profile (AP) scale was developed to assess body condition of Bewick’s Swans in the field. Change in AP score for individual birds correlated significantly with change in their body mass; moreover AP was significantly correlated with condition indices that were designed to remove the potential confounding effects of variation in size between individuals. This demonstrates, for the first time, that mass can be predicted from AP. There was considerable overlap between the values of adjacent profiles however, indicating that predictions for single birds may not be very precise. Generalised linear models were used to examine the variation in AP between birds; variables which statistically accounted for some of the variation included month, year, sex, status, dominance-rank and the multiple effects of month times year, and month times status. Females had significantly higher AP scores than males throughout the winter. The effect of social dominance upon AP was opposite for the two sexes; low-ranking single males had higher AP scores than higher-ranking paired males, whilst low-ranking single females had lower AP scores than higher-ranking paired females.

Bowler J.M., ARDEA 82 (2): 241-248.