Black Woodpecker: iris mark

A high percentage of Black Woodpeckers Dryocopos martius show irregular shapes and marks in the eye. In particular, this concerns a pear-shaped black mark on the iris (see photo here and above in the blog banner). This is found on both sexes and across the global range; it is not confined to any one local population. Furthermore, no other woodpecker seems to exhibit this to the same extent. It is unclear what the function, if any, of this feature is. I will have a note on this subject in an upcoming issue of the UK journal British Birds and will post here when it is published.