US CBP in YUL
 

Traveling to Montreal in February. Return flight to US is scheduled for 5:20am. Obviously I would assume that they wouldn't schedule such an early flight if services weren't available, but just concerned that there will be enough time to check a bag, get through security, and then passport control in time. Anyone know when Delta's check-in desk opens and when security and CBP start work? Thanks.

This will be the first flight out of the transborder area. Obviously, if the departure is scheduled for that time then check-in, security & immigration services will be available - likely with zero wait time. I believe CBP opens at 5am and DL check-in probably 1h prior to departure.

Yup. I've taken this flight before. There was a huge delay at check in, but everyone was there and ready to go!