The Importance of Body Reserves Accumulated in Spring Staging Areas in the Temperate Zone for Breeding in Dark-Bellied Brent Geese Branta b. bernicla in the High Arctic

Seasonal changes in body mass of Arctic-nesting Brent Geese are characterized by a depression in early April, followed by a sharp increase during pre-migratory fattening. Both male and female Brent Geese increased their body mass by 25-35% in the Wadden Sea in April/May. Paired females reach on average higher body masses than single females. Female Brent Geese that returned with offspring to the wintering quarters in western Europe had on average been heavier at spring departure from the staging grounds in the Wadden Sea, than those failing to raise offspring. This difference remained significant after correcting for body size (i.e. wing length). In males, corrected spring body mass had no effect on subsequent breeding success. Measurements of changes in body mass of incubating female Brent Geese, coupled to estimated flying costs (based on data on body mass, wing span and total wing area) showed that body reserves stored in the Wadden Sea were insufficient to accomplish both migration and successful breeding. Presumably the geese refuelled in the White Sea, in order to complete their spring migration and arrive on the breeding grounds in Taymyr in the condition necessary for successful breeding.