Swans are generally monogamous. The reason: because it works for them! They do the heavy lifting together and both have a higher chance of raising young. It helps if you don’t have to spend time and energy every year to find and seduce a partner.
They can sleep on the water and even swim with one leg up
It strikes me that they are often synchronized in action.
White swans look similar, but there are differences.
The mute swan on the right
Trumpet Swan on the Left.