Why mute swans called mute swans?

Well, first the colour. Black swans are ... well... black with a bright red beak and mute swans are white with a black band above the beak and the beak is a orange melting into a red near the tip. Second, mute swans tend to be much quieter than other swans. Mute swans are also one of the heaviest flying birds. Unlike black swans they tend to also be more territorial.