There was a tiny amount of light, enough for me to use manual focus to find the right focus point. And once I did that, I just did a long exposure (1 second in this case)

1 second would be WAY too long for most birds, because they don't sit still, but Frogmouths are ambush predators, that sit perfectly still for hours on end until prey comes close. Also, the lens and the camera body are image stabalised, so I was able to hand hold the camera (with great care) to make it all work.

1 second exposure is enough to let all the light in and some more.