Sexual dimorphism describes the differences in appearance between male and female members of the same species, a common trait among avian species, including ducks. The general rule for most duck species is that the male duck, or drake, is typically larger and heavier than the female duck, or hen. This size difference is one of several characteristics that distinguish the sexes.
The Rule of Sexual Dimorphism
Sexual dimorphism dictates that drakes generally have greater body mass, length, and overall physical dimensions compared to hens. For example, the common Mallard drake typically weighs between 1.5 to 3.5 pounds and can reach up to 30 inches in length. While the hen falls within the same weight range, she is often at the lower end, measuring 20 to 23 inches. This size disparity is consistent across many breeds, but the difference is often less dramatic than in other bird families, and in some domestic breeds, the sizes are nearly identical.
Telling the Difference Beyond Size
Since physical size can be subtle, other characteristics offer a clearer way to differentiate drakes from hens. The most noticeable difference is plumage, especially in wild species like the Mallard. The drake exhibits bright, elaborate breeding colors, such as the glossy iridescent green head, contrasting with the hen’s more muted, mottled brown plumage. This cryptic coloration provides better camouflage for the hen while she is nesting, a survival advantage.
Another reliable visual cue is the presence of a curled tail feather, often called the “drake feather,” found on the male’s tail after maturity. While both sexes possess the iridescent wing patch known as the speculum, the male’s is often a more vibrant blue. Vocalization is also a key differentiator: the hen produces the loud, recognizable “quack” sound, while drakes have a much quieter, deeper, and raspier call, often described as a low rattle or burp.
Biological Drivers of Duck Size Differences
Sexual dimorphism in ducks is largely explained by the pressures of sexual selection and natural selection. The larger size and brighter plumage of the drake evolved due to intense male-male competition for mating rights. A larger body size confers an advantage during aggressive encounters, leading to greater success in securing a mate. The vibrant colors serve as a display of genetic quality used by females when choosing a partner.
The hen’s comparatively smaller size and drab coloration result from different selective pressures related to nesting and survival. A smaller body requires less energy and allows the hen to sit more discreetly on the nest, reducing the risk of attracting predators. While the male’s traits are driven by competition, the female’s traits prioritize fecundity and the protection of offspring.