This is just how drakes are, while not all but most drakes do this. This is a concern to anyone who has a drake near chickens. Drakes do this to any female may they be chicken or duck, male ducks are just violent when mating. They may be targeting the silkies because they are calm and due to the feathers on their head have a reduced range of sight so they are easy to sneak up on. You may need to just separate your ducks from your chickens if it's this bad, the hens not even being able to walk around without being attacked. Make sure the drakes haven't hurt any of the chickens since ducks have different anatomy then chickens a drake mating a hen can injure them and even be fatal.