I was asking myself that very question about an hour ago. The ability is not written as clearly as I might like. Normally, supernatural abilities require a standard action, but does the harpy's captivating song require a standard action each round to maintain the song's effect, much like concentration to maintain a spell does? Also, what does it mean that a captivated victim standing within 5 feet of the harpy offers no resistance? Is the victim helpless? Can the harpy execute a coup de grace in this situation?