This is a May 2014 aquatic warbler video from Poland.
From BirdLife:
The majority of Aquatic Warbler Acrocephalus paludicola, migrating from their breeding grounds in Europe, use “stopovers” in wetlands in north-western France to build up their strength before their onward journey to Africa.
The authors of the paper Autumn migration route of Aquatic Warbler (Ibis 148, 735-743) identify the Seine and Loire estuaries as priority sites, with some smaller wetlands in Brittany receiving large numbers when eastern winds predominate at the peak of migration.
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