Knife - Perfect for duck hunting?

Good day there, everyone! As a past time, I do some blacksmithing, and some bladesmithing too. Money's tight saving for university, so I've elected to craft my presents this year for christmas. My brother in law is an avid duck hunter, and I've never tried it. I want to make him a custom knife. From all the duck hunters out there, what makes a good knife for duck hunting? I can't even begin to image what it would have, how big it should be, what the handle should be, etc. Any help would be appreciated, thanks a lot!