He doesn't care. He's very confident, and critical comments roll off him like water off a(n) ..... back. (*) eagle's (*) bird's (*) duck's (*) turkey's
This is a companion discussion topic for the original entry at https://english.best/questions/20534,he-doesnt-care-hes-very-confident-and-critical-comments-roll-off-him-like-water-off-an-___-back/