Thread creep (excuse for old jokes).The boatmen on the Scilly Isles tease the grockles (tourists) that you can tell a cormorant from a shag because one has 12 feathers in its tail and the other has 13.
If anyone asks you the difference, of course, the acceptable answer is: "I don't know, it's a long time since I had a good cormorant".
RtS (what did you expect? Jack Benny?)