Hawk covers a broad list of species, which have different tactics that they have evolved to use to their own benefit. These could range from the classic swoop from the sky attack, to the set on a perch and drop a few feet to attack, even the land and chase down on foot attack. Depends on species, terrain, time of year, and lots of things. A lot of US hawk attacks on chickens come from coopers hawks, which can use any of these tactics, as well as their graceful slalom flight through obstacles, sometimes perching horizontally on a small tree trunk to get their bearing and timing correct, or even grab a limb and swing around like a pole dancer to make a direction change. Hawks are migratory at various stages of their life cycles, if they grew up eating chickens, and see your chickens as they pass through it is like seeing the golden McDonalds arches on a road trip. So it can happen without warning, usually fall migration. That is how it happens for me, it's always a passer through, the local hawks know that they are no match for my chickens and they don't want to end up broken and bald after tangling with a hen that heard her chick make the wrong noise. But the migrators don't know that these aren't ordinary chickens.