She's the reason strapping young men are willing to crack a rib on the gridiron.

She's the reason fans are willing to miss a play now and then.

She's cheerleader: Hear her roar.

She's 23-year-old Hamilton, N.J., native Kellie Fink - and the 400 or so other young women who turned up yesterday at Lincoln Financial Field for an open audition for a spot with the Eagles' cheerleaders.

"I've been dancing since I was 3," said Fink, who graduated from St. Joseph's University in May with a degree in marketing. "I danced for St. Joe's dance team, and I was captain last year, so I wanted to take it to the next level."

The next level began yesterday with a rapid-fire audition in front of two panels of judges. The dancers were taught a 30-second cheer that they had to reproduce in teams of five. Watching each faux squad do its thing - to the thumping beat of Lady GaGa's electropop tune "Poker Face" (mercilessly repeated more than 100 times during the course of the day) - made one appreciate the final, polished, professional product all the more. The dancers moved as if they lived in their own time zone.

Nothing was in sync, except their obvious anxiety.

Eagles director of cheerleading Barbara Zaun said the open call was the primary way to recruit talent for her squad, whose 38 members stay for an average of three years. The gig, she explained, is strictly part time, paying on average $400 to $1,000 a month. "We have nine definite openings, but our current cheerleaders will need to compete to win their spot back - so there are actually 38 openings," said Zaun, who cheered for the Eagles for three years before becoming director in 2002. "No one gets a free pass."

She said 100 would be invited to compete against one another and the current squad at the semifinals Wednesday. The public can buy tickets to watch the 60 finalists compete April 21 at the Prince Music Theater.

It's daunting to audition every year, said five-year cheerleader Amy Mecca, who was the cover girl on last season's Eagles cheerleaders calendar. "It's definitely nerve-racking for us, but I think it's a good thing. . . . You don't want to get lazy, and you want to keep the girls that have good attitudes."

Mecca, who lives in Northern Liberties, bucks the cheerleader stereotype. A 2006 Drexel University graduate, she's an operating-room nurse at Methodist Hospital specializing in orthopedic surgery.

One judge, Janet Harding, who retired from the squad in 2008 after 10 seasons, said she judged the girls as much for their brains and their get-up-and-go as their beauty.

"We want someone who has the overall package. We look for a personality and for confidence, because you are going to be representing the team in public," said Harding, a real estate agent.

She said the girls wouldn't last if they came on board looking for a husband - or a hookup. Fraternizing with players is strictly verboten. "We lead totally different professional lives from the players," Harding said.

For all the glamor cheering evoked in high school, in the big leagues it's a strictly professional affair. Cheerleaders are hired to represent "the Eagles brand," as Zaun put it. It's a lesson not lost on the newbies, who behaved as if they were on a job interview rather than an angst-ridden audition for American Idol. They have no illusions.

"I want be a forensic psychologist," said West Chester University junior Jessica Losinger, 20, who auditioned with her Coatesveille High School best friend, Megan Maguire.

Well, the romance isn't entirely absent. "I'm actually going to become a writer and pro cheerleader," said New York University junior Camile Shaikh, 20, a San Francisco native and Eagles fanatic.

"If I'm not going to make the Eagles this year, I will be back," Shaikh said with resolve.

Not everyone was there to carve out the perfect career. Take Adrienne Allen, a 31-year-old high school music teacher and single mother from Horsham.

"It's been a lifelong dream, and I've always said I'm going to" audition, Allen said. "But I decided to do it to teach my [students] a lesson about taking charge of their lives and having confidence in themselves to compete in the real world."

So what will it be like to face her students tomorrow? "It's going to be awesome, because I know all of them are going to wonder what happened," Allen said.