▁Europe , ▁a ▁san ctu ary ▁for ▁fish , ▁m oll us cs , ▁cr ust ace ans , ▁and ▁especially ▁to ▁birds ▁that ▁stop - over ▁on ▁their ▁migration ▁between ▁northern ▁Europe ▁and ▁Africa . ▁It ▁is ▁the ▁largest ▁est u ary ▁in ▁western ▁Europe , ▁with ▁about ▁, ▁and ▁regularly ▁hosts ▁ 5 0 , 0 0 0 ▁winter ing ▁water f ow l ▁ ▁Character ization ▁ ▁The ▁Nature ▁Reserve ▁was ▁established ▁in ▁ 1 9 7 6 , ▁covering ▁an ▁area ▁of ▁, ▁character ized ▁by ▁an ▁extensive ▁surface ▁of ▁est u ar ine ▁waters , ▁vast ▁fields ▁inter se cted ▁by ▁cre eks , ▁mar sh es , ▁salt ▁fl ats ▁and ▁all uv ial ▁agricult ural ▁land ▁( mar sh lands ). ▁ ▁Not ▁exceed ing ▁ ▁above ▁sea ▁level ▁and ▁a ▁depth ▁of ▁, ▁distributed ▁over ▁the ▁count ies ▁of ▁Al co ch ete , ▁Ben av ente ▁and ▁V ila ▁Fran ca ▁de ▁X ira , ▁the ▁reserve ▁falls ▁mostly ▁in ▁the ▁up stream ▁area ▁of ▁the ▁est u ary ▁of ▁the ▁Tag us ▁which , ▁with ▁an ▁area ▁extending ▁over ▁about ▁, ▁is ▁the ▁largest ▁in ▁Western ▁Europe . ▁ ▁Ar ound ▁the ▁est u ary ▁a ▁salt mar sh ▁has ▁developed ▁which ▁is ▁a ▁zone ▁of ▁high ▁product ivity ▁of ▁pol ych a etes , ▁m oll us cs ▁and ▁cr ust ace ans , ▁is ▁an ▁important ▁bre eding ▁area ▁for ▁various ▁fish ▁species . ▁However , ▁it ▁is ▁the ▁water ▁birds