How Many Aquatic Warblers Acrocephalus paludicola Stop Over in France during the Autumn Migration?
Résumé
The autumn world population of the endangered Aquatic Warbler Acrocephalus paludicola probably numbers
between 23 000 and 69 000 individuals, including 5 000 to 44 000 first-year individuals, depending on variation in breeding
success and post-fledging survival. After breeding, the species migrates as early as August along a westerly route
along French coast to reach its African wintering grounds. In 2009, French ringers have carried out targeted mist-netting
to enhance the capture of the species, using tape luring in suitable habitats. Overall, 874 different individuals were
captured in France in that year. In 2010 similar ringing effort allowed the capture of 646 different individuals. From this
ringing information, we propose a simple method to estimate the number of individuals which stopped in the country
during the autumn migration, considering all birds or first-years only. Splitting the country in two parts (northern and
southern), the method uses the total number of captures and the number of southern recaptures of birds first ringed in
the north. Overall, we estimated that between 24 000 and 30 000 individuals — most of them in their first calendar year
— stop in France each year during the fall migration. These estimates suggest that probably all first-year Aquatic
Warblers migrate by this western flyway and stop in France to refuel, while adults may partly use a different flyway or
may stop in France, but for shorter times or at fewer sites. The proposed figures highlight the importance of maintaining
suitable refuelling habitats for the species all along coastal France.