When Chip Kelly left Oregon in January to become head coach of the Philadelphia Eagles, he became the third Pac-12 head coach since 2010 to make the direct leap to the same job in the NFL.

The reaction in the league? Nothing to see here. Keep it moving. Kelly continues a migration that began in the early 1970s and, though it slowed at times, has never stopped. The shortest road for any FBS head coach to the NFL is through the Pac-12. In fact, no other conference even comes close.

Kelly is the league's 15th head coach to make the move from Saturday to Sunday since Tommy Prothro moved crosstown from UCLA in 1971 to coach the Los Angeles Rams. In that same amount of time, the Big Ten has sent one head coach to the NFL.

In fact, extend the time frame from the '70s to the entire postwar era and the number of Big Ten coaches who went straight to the NFL increases by only one. If you want to win a bar bet, you don't even have to bother with the names of the two coaches. Ask the sucker to name the school.

That would be ne'er-do-well Indiana, which lost Bo McMillin to the Detroit Lions in 1948 and Sam Wyche to the Cincinnati Bengals in 1984.

The SEC hasn't done much better. The SEC has sent three head coaches to the NFL, and two of them, Nick Saban and Steve Spurrier, have hightailed it back.

OK, before we go any further, here are the ground rules. The head coach had to leave the college and go to the NFL the next season. The school had to be in the conference at the time when the coach made the move. For instance, Jimmy Johnson left Miami for the NFL, but never coached in the Big East. The league didn't form until 1991.

Whatever the criteria, no league has lost coaches to the NFL like the Pac-12. You want to know how open the highway is? In 2007, the Oakland Raiders hired USC assistant Lane Kiffin.

"Pretty overwhelming numbers when you look at it," Oregon State coach Mike Riley.

Just don't ask Riley to explain the numbers.

"In the past, people out on the West Coast were known for either a real balanced pro-style offense or pretty sophisticated passing attack that might lend itself to the next level," said Riley, who left Oregon State in 1999 to coach the San Diego Chargers, then returned in 2003. "… Now, heck, people in the Big Ten are running the spread offense. We don't own any privileges regarding that."

When the run-dominant wishbone offense overtook college football in the 1970s, the Pac-8 never succumbed to its charms. That might be why NFL teams hired five Pac-8/Pac-10 coaches, beginning with Prothro and ending with Bill Walsh of Stanford.

Terry Donahue, who replaced Dick Vermeil at UCLA when Vermeil went to the Philadelphia Eagles, then coached the Bruins to five conference championships in 20 seasons (1976-95), withstood the lure of coaching in the NFL. Donahue rejected the Atlanta Falcons in the late 1980s and, after he retired, the Dallas Cowboys in the late 1990s. He did work for several seasons as director of player personnel and general manager for the San Francisco 49ers.

"I think when you look at the NFL, you're talking about entertainment," Donahue said. "They're in the entertainment business, and they think about it as that. Pro football is always looking for some offensive firepower, and some fireworks."

Dick Vermeil left UCLA in 1976 for the Philadelphia Eagles on the heels of the Bruins' Rose Bowl win over Ohio State. AP Photo

In Los Angeles, where entertainment drives the economic engine of the second-biggest city in the country, Donahue never became more than a sports celebrity. In most cases, the job of head coach in the Pac-12 is just that -- a job. At the most prominent schools in the other conferences, the head coach is an iconic figure.

"That's a whole different lifestyle, almost a different job description in a lot of ways, I think," Riley said. "People always used to say, 'Coach [Bear] Bryant could have easily been the governor of this state [Alabama]' I don't know that I ever heard that about John McKay in California. So I don't know that you can give any credence to the fact that it's about the style of football. … Maybe those guys were just so entrenched with exactly what they were doing and being in that conference was what they wanted to do. It could be as simple as that."

One exception on the West Coast, former Washington head coach Don James turned down the opportunity to coach the Seattle Seahawks in 1982 because he had taken root with the Huskies.

"They were interviewing guys downtown on bar stools: Should I go or should I stay?" James said, laughing at the memory. "I finally did a press conference and said I'm staying."

It could be that universities that share a market with NFL teams lose more coaches to the league. A school such as Boston College, clamoring for attention in a crowded market, might be more liable to hire a prominent NFL assistant coach such as Tom Coughlin, who left the Eagles for the Jacksonville Jaguars in 1994. That best explains why, even without counting Johnson, the 22-year-old Big East has lost six head coaches to the NFL.

That places the Big East a distant second to the Pac-12. Although three coaches have left in the past four years, three former NFL head coaches -- Kiffin, Riley and Jim Mora of UCLA -- are in the league. The Pac-12 leads in that category, too.