Why do ducks have blue wing flashes?

Originally posted on Ray Cannon's travel blog:
Mallard (Anas platyrhynchos) 6 June 2021 Beds, UK. Photo by Raymond JC Cannon 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.…