Is a US passport required for flights out of Canada?
A passport from some country is required to board a transborder (to the US) or International flight. Canadian airline employees need to verify a passport and will check it again at boarding time. US law requires US citizens entering the US to use their US passports even if they have other citizenships. It you are flying from an airport that has US preclearance then the US CBP agents will absolutely require a US citizen show their US passport.