Yes, hawks are a very big threat to all your birds.
I take it that your birds free range during the day?
The best form of defence is to keep them in a covered run at all times but if this isn't possible or desirable and it sounds like it isn't, then you need to provide emergency cover for your hens in case the hawk attacks. These are all fairly simple to set up. These are things such as bushes, branches or boxes in the area that the hens free range in so that they can "run for cover" in the case of an attack. Any roosters in the flock could act as early warning systems..
On a completely different level is buying another animal such as a llama, or certain breeds of dogs that will be with the hens constantly, acting as guardians to them.
Let us know how you get on.