[1667] Der Eider, des -s, plur. ut nom. sing. S. Eidergans.
Brockhaus-1809: Die Eider
Brockhaus-1911: Stellersche Eider · Eider
Herder-1854: Eider
Meyers-1905: Eider [2] · Eider [1]
Pierer-1857: Eider