Nobody could quite figure out how a Muggle ghost had managed to find its way into Hogwarts. Some suspected Peeves had a hand in it, judging by the way Filch could be heard venomously muttering about "taking in strays" for days after its arrival. The newcomer did seem to conspire with Peeves on occasion, true, but more often than not he'd take matters of mischief-making into his own hands: teaching bawdy limericks to the first-years, interrupting Transfiguration lessons by belting out drunken sea chanties, always vanishing with a cocky grin and a tip of his hat before he could be reprimanded.
Even while airborne, his movements had an oddly loose, rolling quality to them. His fingers constantly fluttered about like the birds that had given him his name, and one could hear the jangle of his many hair trinkets long before he appeared. A few students even swore you could catch a faint whiff of saltwater in his wake - though those students usually ended up being laughed at by the haughtier Ravenclaws for their stupidity. (Without fail, the same Ravenclaws would later be spotted giving the air a curious sniff as he passed by.)
He would only share the stories of his seafaring adventures when asked, and always with a reluctance betrayed by the spark in his shadowed eyes. They were spectacular, grandiose tales, full of blood and mystery, and exaggerated with wild abandon whenever it suited him best (much to the disgust of the other ghosts, especially Nearly-Headless Nick). He never lacked a captive audience, and it wasn't long before his fame spread throughout the school, his stories enthusiastically retold over breakfast each morning. That pleased him immensely.
Occasionally, though, Hagrid would find him sitting on the shores of the lake, a ghostly bottle clutched in one hand as he wistfully stared out at the water. "All righ' there, Cap'n?" he'd call. That was the one thing he'd always insisted on: it was Captain, mate, Captain if you'd please.
He'd flash him a quick, if somewhat sad, smile. "Aye, mate, m'fine" he'd call back, and soon afterward he'd drift back to the castle, humming a long-forgotten tune under his breath as he drained the last of his rum.