Mallard

The mallard is 56–65 cm long, has a wingspan of 81–98 cm, and weighs 750–1,000 g. The procreation male is distinctive, with a green head, black rear end and a yellow bill tipped with black. The female Mallard is light brown, like most female dabbling ducks. However, both the female and male Mallards have dissimilar blue speculum edged with white, important in flight or at rest. In non-breeding (eclipse) plumage the drake becomes drab, looking more like the female, but still noticeable by its yellow bill and reddish breast.

The Mallard is a rare model of both Allen's Rule and Bergmann's Rule in birds. Bergmann's Rule, which states that polar forms tend to be bigger than related ones from warmer climates, has many examples in birds. Allen's Rule says that appendages like ears lean to be smaller in polar forms to diminish heat loss, and larger in steamy and desert equivalents to ease heat dispersion, and that the polar taxa are stockier overall. Examples of this rule in birds are rare, as they lack exterior ears. However, the bill of ducks is very well supplied with blood vessels and is defenseless to cold.