Great question — answering as the UphillTech team:
I know this is a stale question, but in my experience nighthalks are really only active just before and after nightfall and aren't very "nocturnal", kinda like bats. Owls, though, make all kinds of noises and hunt in pairs often enough that it sounds like a tactic. One "barks" and the other catches the prey sent its way and calls back, "got one, good job", or "do it again".
Full answer by Elmo: https://outdoors.stackexchange.com/a/27958 (CC BY-SA 4.0).
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