Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Confusion; Communication Breakdown
Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Environment - Non Weather Related; Staffing; Human Factors
While taking Runway X for departure in ZZZ; we saw a single engine tail wheel GA aircraft climbing on the upwind leg and turning out to the north around midfield after taking off the opposite direction on Runway YY. We were unaware of this departing traffic until seeing them airborne at what I would guess 200 feet AGL. I stopped the aircraft around the runway numbers when I saw the airborne aircraft and we had not initiated our takeoff roll. They were most likely off the ground by the time we taxied onto the runway; but we could not see all the way to the opposite end due to runway slope. The GA aircraft made a CTAF radio call telling us he had self announced his intersection departure off Runway YY. We told them we were unaware they were taking off the opposite direction.The airport was uncontrolled at this time of day due to reduced Control Tower hours of operation. We were monitoring CTAF frequency while holding short of the runway and simultaneously getting our IFR clearance from Center. I remember hearing a call on CTAF but couldn't clearly understand it. This was further hindered by listening to both radios at the same time. I don't recall hearing any previous CTAF calls such as this other aircraft self announcing their taxi. When the other aircraft called us after taking off; my First Officer mentioned he believed to have heard the aircraft announce departure off Runway XX. The aircraft stated he announced an intersection takeoff for Runway YY. Main contributing factors were an airport that has enough traffic volume to require a Control Tower not having their Tower open; nice VFR weather for GA flying; the GA pilot not being clear about their intentions over CTAF or speaking up when we announced our takeoff intentions for the opposite runway; calm winds allowing for takeoff from either direction; and not pausing as a crew before taking the runway to verify we understood the unclear CTAF call and would be clear of conflicting traffic.I would assume the Control Tower is running a reduced schedule due to less air traffic than normal given the current coronavirus climate. It would be better if they were open at this time in the morning when Part 121 flights are scheduled. Self announcing on CTAF is everyone's responsibility at an uncontrolled airport. The GA aircraft could've been clearer about their intentions and made more calls. As a crew; we did our part by announcing our intentions; and that should  caught the attention of the opposite direction traffic. The only remaining factor that could've prevented this potential runway incursion would be for us to use good CRM by each focusing our listening attention on individual comm radios during situations requiring monitoring of both; and ensuring to verify that any unclear radio calls are not coming from any potentially conflicting traffic.
Captain reported a GA aircraft departed in opposite direction while they were on the runway ready for takeoff. The Tower was closed at the time of the event.