An annual study found Delta Air Lines is the stingiest among U.S. airlines in making mileage award seats available for frequent fliers to book.

The study by Shorewood, Wis.-based IdeaWorks showed that Southwest Airlines was the U.S. carrier with the best availability of frequent flier award flights.

Delta did improve availability of domestic award flights, according to the study, including introducing a 10,000-mile one-way option for some flights.

“We’ve worked really hard in trying to keep our commitment to opening up more seats on the lowest level,” said Karen Zachary, Delta’s SkyMiles managing director of global programs. “We used to be really bad.”

With the 10,000-mile awards and other systems such as Southwest’s frequent flier program requiring points for a flight based on the fare, “The 25,000 mile round-trip award fare that historically has been a standard for the airlines, I think you’re going to see that completely change,” Zachary said.

The IdeaWorks study is based on booking queries on frequent flier program websites for “saver” awards, or 25,000 points. It found 100 percent award seat availability on Southwest, versus 57.9 percent availability on Delta.

Delta this year changed its frequent flier program to award miles based on dollars spent instead of distance flown. It also removed from its website the charts that show customers how many miles are required for award flights. Instead, frequent fliers can view an award calendar while searching for flights in the booking engine.