Changes in body mass and hormone levels between wintering and spring staging areas in dark-bellied brent geese Branta bernicla bernicla
Résumé
Wintering migratory geese generally replenish protein stores lost during migration before building up their fat stores (Gauthier et al. 1992). Dark-bellied brent geese Branta bernicla bernicla winter mainly along the coasts of France, England and the south-western part of the Netherlands, and stage from March till late May mainly in the Wadden Sea, stretching from the northern Netherlands to Denmark (Ebbinge et al. 1999). During spring, geese accumulate body stores needed both for their long-distance migration to the breeding grounds and for reproduction (Spaans et al. 1993, Ebbinge and Spaans 1995). Dark-bellied brent geese, migrating about 5000 km between the wintering and breeding areas (Green et al. 2002), increase their body mass by 25/ 35% on spring staging areas in April and May prior to migration towards the breeding grounds (Ebbinge and Spaans 1995, Spaans and Postma 2001). In addition, females that were heavier at spring departure prior to breeding are more likely to return the following autumn with offspring to the wintering grounds than those that did not manage to achieve high body masses in May (Ebbinge et al. 1982, Ebbinge and Spaans 1995). This suggests that deposition of energy stores by females in spring is a major factor influencing breeding success (Ebbinge 1989, Spaans and Postma 2001). Intermediate stop-over sites between theWadden Sea and the Siberian breeding grounds on the Taimyr peninsula have been discovered lately along the White Sea and the north coast of the European part of Russia (Ebbinge and Spaans 1995, Green et al. 2002). We think, however, that the staging periods on these stop-over sites, though essential for the birds to top up once again, are too short for birds to compensate for a poor start from the Wadden Sea.