Early departure of Shelduck Tadorna tadorna families from Chausey archipelago: new explicative data

摘要

The Chausey archipelago is a breeding site of national importance for Common Shelduck Tadorna tadorna. Chicks are known to leave the archipelago with their parents the day following their birth, to reach the Bay of the Mont Saint- Michel (BMSM), 30 km southward. A first hypothesis assumed that trophic resources, and more precisely the scarcity of Annelida polychaeta Hediste (Nereis) diversicolor, could explain these movements. The results presented in the present paper, including benthic macrofaunal and bathymetric feature analysis of the two sites and hydrodynamic analysis of the Breton-Normand Gulf, bring into question this first hypothesis. The benthic macrofauna of the feeding grounds of the two sites is similar, and the abundances are even higher in Chausey than in the BMSM. Moreover, H. diversicolor is more abundant in Chausey (the Search highest densities reaching more than 600 ind.m(2)). The spatial characteristics of the intertidal sand flats of Chausey seem to be more relevant to explain theses movements. In addition, if the arrival of the chicks in the BMSM can be favoured by the hydrodynamic conditions of the Normand-Breton Gulf, the families have to swim, what confirms the current hypothesis of a site selection by the birds.