Delta Air Lines is returning to the old practice of offering bonus miles for bookings on its Web site.

Atlanta-based Delta is offering its frequent flier members 500 bonus miles for roundtrip bookings at through May 31. Frequent flier members at Delta’s merger partner Northwest can also earn 500 bonus miles for bookings at Northwest’s Web site.

It’s part of Delta’s effort to increase bookings on its site to lower costs. The company aims for 40 percent of its North American bookings to be made at its Web site this year.

Delta’s move comes after online travel agencies including Travelocity, Expedia and Orbitz waived their booking fees through May 31. As travel drops amid the recession, travel companies across the industry are offering promotions to stimulate business.

Selling tickets through an online travel agency costs Delta extra because airlines pay other companies to distribute their products.

In an e-mail to frequent fliers, Delta touted its site’s best-fare guarantee and noted that its Web site does not charge ticketing or booking fees.

Delta started offering bonus miles for bookings at its Web site back in the 1990s to encourage customers to try buying tickets online, but ended the incentive by 2007.