"Cornish" is a designation for something that originated in Cornwall, England. Cornwall is a region on the south west corner of England, historically an economically poor area with few natural resources. Cornwall is almost a pennisula, bordered on 3 sides by rocky coastline. When times are hard people will eat most anything, seagulls are fat, plentiful, and easy to catch using a fishhead for bait.
The only problem is that no one wants to eat "seagulls". But we have a time-honored tradition of renaming undesirable foods and then passing them off as "gourmet". Such as Escargot (snails), Squab (pigeon), Calamari (squid), Mountain Oysters (testicles), Salisbury Steak (hamburger), Truffles (fungus) etc. Just rename those pesky seagulls to "Cornish Game Hens" and viola!