The speculum is a patch of often iridescent color on the secondary wing feathers of most duck species. It is often seen as a bright patch of color on the rear of the wing when the wing is spread during flight or when the bird is stretching, preening, or landing.

What does the blue feather on a duck mean?

A very common example is the mallard (Anas platyrhynchos), with the blue-coloured speculum of both the male and female, and the green head feathers of the male. The iridescent colours arise from light interacting with stacks of melanosomes, melanin-pigment containing organelles, present in the feather barbules.

Do female mallards have blue feathers?

Both male and female mallards have distinct iridescent purple-blue speculum feathers edged with white, which are prominent in flight or at rest but temporarily shed during the annual summer moult.