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Most large Canadian airports allow Nexus members to use Nexus kiosks and skip the regular lanes. This has been the practice for a number of years. But unlike at the land border, you still need a passport. THat's because airlines require passports when taking international flights. THere have been some instances of Nexus members occasionally being allowed to board on the strnegth of their Nexus card alone (I've done it myself), but it is not standard practice, and you could be in a big jam if the airline says no, which is most likely.

With regard to your second question- yes you can re-enter Canada without a passport IF you travel by road in a personal vehicle, and if you carry proof citizenship such as a citizenship card or Nexus card or expired passport. These things will not be enough if traveling by air, train or bus.