From what I've read.....
Good to start their first free ranging foray late in the day,
maybe an hour or less before they normally go to roost,
they probably won't go far and then you can see that they'll come back.
Subsequent days start a bit earlier, and so on and so forth.
Predator risk will likely always be there, whether you're 'watching' them or not.
It's why my birds stay confined in a mesh covered run.
How big is your run @aart ? and how many chickens? I want to add a run since I added chickens and curious
Gary