Many types of dabbling ducks have iridescent wing patches called specula (singular: speculum). These flashes are usually blue or green, although the colour can vary somewhat according to the angle and the lighting conditions. Ducks can also control the extent to which these flashes are visible when their wings are folded (see below).

In Mallards (Anas platyrhynchos), which are probably the easiest duck to photograph in the UK, the flashes are blue in both males and females. The blue patches occur on the secondary feathers and are very apparent when the duck stretches its wings (below) and here.

The colours are structural rather than pigmentary…