If you are saving up reward miles to pay for a trip on Delta Air Lines, you might want to book the trip before the airline switches to a new format that could raise the number of miles you need for the most popular destinations.

For travel on or after June 1, 2016, Delta said the number of miles needed to book a flight might rise based on destination, demand and "other considerations."

But don’t expect the airline to disclose the exact reasons why fares will vary from day to day.

Southwest Airlines adopted similar changes to its reward program, taking effect last April.

The move — the latest by airlines to reduce the value of reward points — is sure to frustrate travelers, who are already relying less and less on loyalty reward programs to pay for airline tickets.

"What’s disturbing about this is the complete lack of transparency," said Tim Winship, publisher of the rewards program monitoring website Frequentflier.com. "Nobody knows what these other variables might be."

Several recent surveys have shown that confusing rules and stipulations hidden in tiny print are among the biggest frustrations travelers have with loyalty programs.

That may explain why a 2013 survey by Deloitte found that travelers ranked the importance of loyalty reward programs 19th among an airline’s best attributes, behind comfort, staff attitude and check-in convenience.

Delta spokesman Anthony Black defended the reward program, noting that many flights won’t rise in cost because of next year’s changes.

Other airlines may consider converting to Delta’s new system but not until they see if the changes are a success, said Brian Karimzad, director of the reward monitoring website MileCards.com.

Expect more add-on airline fees

Charges for checked bags, food and seats with extra legroom, among other extras, generated $38.1 billion for the world’s airlines in 2014, a 21 percent increase from the previous year, according to a report from IdeaWorks, an airline consulting firm.

But you can expect such revenue to continue to soar as airlines consider adopting even more fees.

How about a fee that ensures your bag is delivered at the baggage carousel before all the others?

Or a charge to let you be the first to exit your plane?

How about a charge to put a radio frequency identification tag on your luggage so you can track its location in real time from your smartphone?

These were among the extras that at least 60 percent of travelers said they were willing to pay for, according to a survey of 2,339 business and leisure travelers by Flightview, a Boston-based flight information firm. An even higher rate of millennials, 75 percent, said they were willing to pay such fees.

Among other fees travelers would be willing to pay, according to the survey, are flight status alerts that are automatically sent to family, employers, hotels and others; a smartphone app to let you preorder food and drinks at airport eateries; in-seat charging stations for mobile devices; and a rental car service to pick you up at the terminal curb.

Some airlines already offer a few of these services, and others may be ready to adopt them in the future, said Flightview spokeswoman Katherine Wellman.

"We just wanted to pass along to airlines what people really want," she said of the survey.

United Airlines pays out ‘bug bounties’

In a first for a U.S. carrier, United Airlines has paid out "bug bounties" to cybersecurity experts who found and exposed weaknesses in the airline’s website.

Two cybersleuths were each paid 1 million loyalty reward miles for uncovering gaps in the airline’s Web security.

The Chicago-based carrier announced it would pay out the bounty in May, a few weeks before the latest of several technical glitches grounded flights for nearly 90 minutes.

United officials say the "bug bounty" program was an idea that the airline borrowed from technology companies in Silicon Valley that also offer rewards to anyone who can identify cybersecurity gaps.

At United, the bounties are paid on a sliding scale based on the severity of the security gap, with 1 million reward miles paid to whoever can find an opening that allows someone to execute computer codes at the United website from a remote server.

That reward should pay for about three first-class round-trip tickets to Europe from the U.S.

Hugo Martin, Los Angeles Times