Bewick's swans Cygnus columbianus bewickii, an analysis of breeding success and changing resources

Bewick's swans in W. Europe changed from aquatic feeding to field feeding around 1968. Parameters for breeding success were compared for the periods prior and after the changement to field feeding. Proportions of juveniles and of parental birds among birds of breeding age are positively correlated with temperatures at the start of the breeding season near the Barents Sea. This may be explained by the number of snowfree nesting places available at the right time for successful breeding. Brood size of successful breeders does not show this correlation but was significant higher before the birds changed to field feeding. This is ascribed to lower sizes of clutches, which may be the result of decreased capabilities of laying on body nutrient reserves during the winter season. The latter conclusion is mainly still an hypothetical one.