The swans, the largest of all waterfowl, are known for their beauty, elegance, and grace. Of six species of swans in the world, four are found in the northern hemisphere, mainly North America and Eurasia. and two in the southern hemisphere. The four northern species, the Tundra, Mute, Whooping, and Trumpeter Swans, have all white plumage. In South America we find the Black-necked Swan with a black neck and white body and in Australia there is the all-black Black Swan with white wingtips.
The white swans of the northern climes nest on the ice-and snow-covered tundra and are thus camouflaged. Newly hatched birds are gray, presumably because by the time they hatch the snow is mostly gone and the gray rocky ground of the tundra appears. But in the breeding habitats of the Black and Black-necked Swans, there is no tundra, so selective pressure to keep them all white is absent. But still, why did evolution give them the black coloration?
The smallest member of the swans, the Black-necked Swan, is found in freshwater marshes, lagoon and lake shores in southern South America, breeding in the Chilean Southern Zone, Patagonia, Tierra del Fuego and on the Falkland Islands. Black-necked swans are completely white except for their black neck and head. A white stripe runs across each eye. The bill is gray with a red knob near the base; the legs and feet are also red. The young are light gray in color and have black bills and feet. Given the contrasting colors of red, black and white, one could surmise that since camouflage is not an issue, the coloration is for courtship or territory defense purposes. Arctic breeding swan pairs, because of the cold temperatures, alternate sitting on the nest. Black-necked Swan males, however, rarely do. They spend most of their time off the nest, defending it. The coloration presumably makes them more obvious and thus better defenders.
The Black Swan is common in the wetlands of southwestern and eastern Australia and adjacent coastal islands. Black swans are mostly black-feathered birds with white flight feathers. The bill is bright red with a pale bar and tip; the legs and feet are grayish-black. Again, the red beak may serve as a sexual signal, but like the arctic swans, both sexes incubate. Most interesting is that the Black Swan is all black except for the outer parts of its wings. In many large birds like Snow Geese, Wood Storks, many hawks and cranes, and others, the wing edges are black or dark brown because the melanin pigment that produces these dark colors also strengthens the feathers, and the flight feathers endure the most wear. Perhaps the white feathers, visible only in flight, serve some function in communication.
And then there is the Coscoroba Swan, not a true swan, but a large waterfowl ancestral to geese and swans.
The English word swan, similar to the German Schwan, Dutch zwaan and Swedish svan, is derived from the Indo-European root swen (‘to sound, to sing’). The Swan Song is a metaphor for a final gesture, effort, or performance given just before death or retirement. The phrase refers to an ancient belief that swans sing a beautiful song just before their death since they have been silent (or at least not so musical) for most of their lifetime. You can read some other interesting mythology at The Conversation.