[EBB Sightings] Topknot ducks

Dear EBB;

Can one of the resident experts tell me about the mixed-up ducks that you occasionally see with the topknot of feathers on the head?  They are clearly a cross with a domestic duck, but why the topknot, and is it feathers or bone?  Not having any luck with on-line research.

Thanks,

Pat Bacchetti