The speculum feathers of various ducks are curious. These are the patches of often iridescent colour on the inner wings of many species of ducks. The colour of speculum (Latin for mirror) feathers differs from one species to another, and the function seems a bit obscure.
Many of the other colourful feathers found on male ducks seem to have evolved in response to the sexual preferences of females. But, breeding preference is unlikely to have influenced speculum feathers, for they appear essentially the same on each sex.
Further, the speculum feathers are not particularly visible unless the duck is flying and then they are most easily seen from behind. It seems likely that the function is primarily that of a signal among flying members of a species so as to maintain the cohesion of a flock.
The blue-purple speculum of the Mallard is seen from behind when the bird is in flight.
The speculum seems to help ducks identify their own species when flying in a flock (birds of a feather…).
Another wonderful natural history lesson – thank you, Alistair.
I didn’t realize this. Thank you for the info, and those remarkable photos.