Initially they were overwhelmed by the light, the noise, the fresh air. Soon though they were busy nibbling grass and running around. I put some compost in a kitten litter tray and put it in, and they LOVED it.
When I put them back in their brooder, I put the dirt tray in with them. I popped out an hour or so later to check on them, and they were mostly sparko, with one of them asleep in the tray.
I've just checked the camera, and they are now wide awake and playing in the tray. It was hard to get a snapshot, they were moving so fast: