Decoys work for everyone is a phrase that's often associated with the idea of using swan decoys to scare off other waterfowls. However, some people argue that the idea is nothing more than an old wives' tale. Generally speaking, it's believed that swans are aggressive towards ducks and will attack them if they find themselves in the same waters. So a swan decoy nearby could be enough to scare off ducklings who might be tempted to come near.