Widely distributed throughout the Northern Hemisphere, the Mallard migrates to Japan for the winter, settling on lakes or marshes, even though some breed in northern Japan. The female is plain brown and resembles a Spot-Billed Duck, which is almost the same size. During autumn and winter the male has a beautiful iridescent green or purplish head, while its coloring in summer is similar to that of the female. The Mallard's call is the quintessential “Quack, Quack” of a duck with a hoarse voice.
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All domesticated livestock and poultry originate from wild animals and birds. Domestic ducks are Mallards that were tamed over time by humans. A common white duck has almost the same genes as the Mallard, so they can interbreed. In biology, two different animals that can breed and produce offspring are categorized as the same species. On the other hand, if they cannot, they are categorized as different species. From this perspective, human beings belong to one species even though we have differences in skin color and eye color. Similarly, the Mallard and the domestic duck can be said to be of the same species.
There are domestic ducks that have the same coloring and patterning as the Mallard, so if they escape and return to the wild, these feral ducks can be difficult to distinguish from Mallards. One has to look for characteristic features of domestic ducks, such as larger body size than Mallards, plumper appearance, or lack of fear of humans.
Ducks can be divided into two groups: the first includes dabbling ducks like the Mallard and the Spot-billed Duck that feed at the water's surface, and the second those that dive under the water to feed. Surface feeders eat mainly vegetable matter such as seeds of water plants, while divers eat animal matter such as shellfish in quick succession.
The two groups of ducks, which include more than thirty species, have few genetic differences among the species. As the history of this differentiation of species is short, females of all species resemble each other. Crossbreeding often happens, most particularly between Mallards and Spot-billed Ducks, and hybrids possessing features of both species have been confirmed.
According to the biological species concept, which poses that hybrids of different species are infertile and thus cannot reproduce, species differentiation should be maintained. However, hybrids of Mallards and Spot-billed Ducks can breed, and their offspring can crossbreed over many generations. This is possible because the Mallard and the Spot-billed Duck have few genetic differences. In view of this, it cannot be said that there is no possibility of increased crossbreeding between feral ducks and Spot-billed Ducks.
A male before molting into beautiful plumage. Male ducks have dull, inconspicuous plumage similar to females during summer, but during autumn they molt, and by winter develop impressive colorful plumage to attract a female (The Spot-billed Duck is the exception; the male does not change its plumage).
This picture shows a domestic duck. Some ducks have similar colors to the Mallard, their original species, but those with a large rump are easily distinguished as not being a Mallard.