Southwest Airlines is changing its unusual boarding system by limiting the opportunity to pay an extra fee and jump ahead of other passengers in the race for the best seats.

The airline said Wednesday that it has not dropped “EarlyBird” entirely from any flights, but it is “limiting the number of spots available for purchase on certain flights, routes, or days, as we work on product enhancements.” As a result, the airline said in a statement, the service “is unavailable for some customers looking to purchase it.”

The perk starts at $15 but can be higher, depending on the length of the flight and demand for seats. Limiting the number of EarlyBird spots could push Southwest passengers into other options for moving up in the boarding line, including buying the highest-priced fare, called “Business Select,” which comes with a top-15 spot in line.

The limits on EarlyBird took effect Aug. 15. A spokeswoman for the Dallas-based airline declined to say how many early-boarding spots will be cut, calling that “privileged” information.

Unlike other major U.S. airlines, Southwest does not assign passengers to specific seats ahead of time. Instead, passengers board in the order that they checked in for the flight and pick seats once they are on the plane.

EarlyBird buyers can avoid the rat race and secure a prime spot in the boarding line by checking in before the normal check-in process starts, which is 24 hours before departure. EarlyBird has proven popular among passengers who want a window or aisle seat near the front — and just as importantly, room for their bag in the overhead bins.

During a call with analysts and reporters last month, Southwest executives boasted that EarlyBird fees raised more than $100 million in the second quarter — about 1.4% of the airline’s total revenue.

“EarlyBird had been lagging a little bit through the pandemic recovery,” said Chief Commercial Officer Ryan Green, “but it performed very well in the second quarter.”