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