Jack is or was a male anthropomorphic pigeon.
Jack was the bartender of the “Miner's Bar”, a bar in Dawson City which Scrooge McDuck frequented in his Gold Rush days (foregoing Goldie O'Gilt's Blackjack Saloon). It served a very cheap, "nasty" beverage known as dwarfberry juice, for which Scrooge was the only (albeit consistent) customer. Jack himself appeared to be a friendly, soft-spoken, warm fellow, on good terms with all of his clients — Scrooge included.
Behind the scenes
Jack appears in the 2007 story My First Million.