Speaking of owls, I've been wondering if some big owl took a medium-sized cochin rooster of ours. He'd been roosting on a fence ever since the pullets with whom he used to sleep inside a pen were promoted to the laying flock. I was trying to get him to sleep in with the 4 guinea hens & would place him in there after dark. My concern was that the other roos already established in the laying flocks would beat him up if I added him in there.
I saw him on the fence at his last sunset and planned to move him in later. Sadly, when I went out after dark, he was gone. My son had thought he'd heard "something" at the chicks' pen, so we went out to investigate, and that's when we noticed the roo was gone.
By day all I saw were 2 small feathers on the ground below the fence. I think it was an owl that took him because it was done so neatly & quietly. I think a raccoon, fox, cat or dog would have left a telltale trail of feathers from a struggling bird.
I was sad to have lost this roo, especially since I feel at fault for not keeping him safer. But if he had to go, I feel that feeding a large owl's family to be a rather noble cause.
As for hawks, you may need to learn by trial and error. You have to decide how you wish to balance your chickens' liberty and safety. You might be fortunate to be in an area relatively safe from hawks, you'll have to watch your skies & talk with your neighbors.
To give your chickens more of an advantage over hawks, provide hiding places like tables, benches, even wood sheets propped up on bricks. If you have roosters they should be watching for predators and will make a specific sound when they see a hawk overhead. When they say that bird word everyone stops & looks up in the sky, including me. If they have places to run under cover they will be safer. I also put shiny things around & strung over their pens, foil pinwheels, spinners, old CDs & DVDs, etc, that seems to help.
You can't keep your chickens 100% safe unless you carry them around with you, and even then you might accidently trip & squash them. I just try to do my best to know the predators most prevalent in my area and give my chickens the best advantage over them. I'd rather give them some free-range time (lower feed bill, tastier eggs) without monitoring them (can't spare that time) even if it means losing a few birds a year (it's not even that many here).
I wish you & your birds safety & success, whatever you decide.