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."
When not performing, the pair swapped "funny stories" during lunch breaks and even worked on a duet. "We spent a lot of time one day in his dressing room just playing around. I was playing keyboard and he was playing guitar and we were just trying to work out something, and that was pretty fun," she recalls.
Selena Gomez Visits BackstageAnother highlight? Meeting fellow Jonas Brothers Joe, 20, and Kevin, 22, who came to one of their New York City shows. "They were so nice and down-to-earth," she says. "To just see how normal they are, it was really cool."
Birch also learned about a Jonas family tradition when Joe gave her boyfriend, art director and model Ariel Stark-Benz, a backstage embrace.
"We went to say goodbye and my boyfriend went to shake his hand and Joe gave him a hug. [It was] kind of awkward, and Joe was like, 'We're a hugging kind of family.' It was really cute. Every time I saw them they were hugging and just really friendly."
Nick's brothers weren't the only famous faces to pop up backstage.
Selena Gomez, Nick's one-time – and rumored to be current – girlfriend "was there part of the time," reveals Birch. "She seemed really cool. I think she's pretty close to the family."
Even though the tour with Jonas has ended, Birch hopes to hit the road again, this time to Europe to promote her CD Bible Belt.
Jonas, meanwhile, just released Who I Am, his first album with Nick Jonas & The Administration.