Some birds just have no shame. Yeah, I'm looking at you, pigeons and sparrows. Don't act as if you don't know what I'm talking about: the way you and your minions chase morsels and crumbs on tables and chairs in public spaces, heedless of the humans trying to enjoy a meal. Well, you've met your match at South Station. A few months ago, Cushman & Wakefield, the property management firm that oversees the transportation hub, installed a $300 green gizmo outside the doors to the train tracks. Through a small speaker, the device simulates the trills and calls of predatory birds — hawk, sea gull, owl, and falcon, among them. "It worked instantly," says facility manager Jozef Kovac, who used to shoo offenders out of the concourse like an angry shepherd. After the device was installed, some commuters complained about the noise, so Kovac had it turned off. But in that week of silence, the dreaded pigeons and sparrows returned. Now it's back on, squawking at random intervals during the day at a quieter but still effective volume, a cunning Hitchcockian solution to what South Station officials say was a serious problem.