There aren't many of them, but there are a few: ducks with green feathers.
Most commonly it is the male of the species which carries the colorful plumage. Nobody really knows why the feathers are green. They just are.
Scientists speculate at length about the reason for such unusual coloring in the birds. Often they speculate that the color is for mating purposes, but if you really push the scientists to come clean on this, they will admit they are just guessing.
But for whatever the reason, there are these ducks with green feathers, our beloved green ducks.