An Etymological Dictionary of the German Language/Adler
Adler, m., ‘eagle,’ from MidHG. adel-ar (also adel-arn), m.; prop, a compound, ‘noble bird of prey.’ It is noteworthy that Aar in ModHG. is the nobler term, while Adler serves as the name for the species without any consciousness of its origin from Adel and Aar. OHG. *adal-aro appears by chance not to be recorded. Corresponds to Du. adelaar (besides arend).