aquatic warbler Acrocephalus paludicolais the only globally threatened passerine breeding in Europe.
We studied morphometric traits of176 individualscaptured at one of the key
European autumn stopover sites, the Loire estuary on the Atlantic coast of
France, in relation to age (first-year and older) and sex (based on molecular
determination). We found significant sex differences in wing length, third primary feather lengthand body mass, with males being
longer-winged and heavier than females; adult females also had deeper bills.
However, high overlap in all measurements between the sexes meant the best discriminant functions based on wing
length (all ages) and bill depth (adults) correctly classified only 87%and 75% of individuals, respectively. The
mean potential non-stop flight range of autumn staging birds at the
Loire estuary with high fuel load was estimated at 1,178 km for adults and 926
km for immatures. We conclude that in autumn migrant aquatic warblers probably
do not fly directly to wintering grounds in west Africa. Instead they gain significant body
mass for onward migration at a few key stopover sites in western Europe and the
southern Mediterranean region. Conservation of a series of important refuelling
stopover sites, especially wet grassland habitats, along the migration route
is therefore essential for effective protection of the aquatic warbler.