Mallard beaks range in color from a dull green to a sunny yellow. Females tend to prefer males with bright yellow beaks. Scientists think that females prefer brighter beaks because it signals that the male mallard is fit and able to produce healthy offspring. Beaks are colored due to carotenoids - the same pigments that make vegetables brightly colored. More carotenoids leads to a brighter bird beak. When determining the exact cause of the green coloring, be careful who you get your information from. You see, there are a lot of quacks out there!