They absolutely can just swoop in. Do you have goshawks? I lost two birds two weeks ago in the exact same manner. Caught the bugger on the third attempt, so I know who the culprit was/is. They are super hard to deter because they don't operate like larger hawks and eagles. They are fairly small, and can chase birds through fully covered areas, and through forests. They even give chase on the ground. Nothing short of a covered run can stop them. They often eat the neck and head, then leave the prey. The plan is to come back over the next few days and eat the rest, as chickens are usually too heavy for them to cart off.