Thanks to the teen idol's rabid fans, the retro-pop singer-songwriter, 27, says she witnessed something "similar to Beatlemania" while on the road with Jonas, 17, earlier this year.
"I was really surprised at the craziness of the fans and how insane they are – and how genuinely sweet," says Birch, whose song "Valentino" is on the movie soundtrack to the upcoming Valentine's Day. "They would spend hours in the freezing cold just to catch a glimpse of him."Birch logged some indoor quality time with Jonas, whom she calls "super normal and super sweet." And also very mature: "The first thing that really surprised me was how much of a leader and how much of a showman he was onstage," she tells PEOPLE. Offstage, she says, he's "very quiet and serious." Interesting quotes..