Shareholders of Atlanta-based Delta Air Lines voted to allow the company to expand the size of its board of directors, but turned down a measure that would have required senior executives to retain more stock during their tenure.

The airline told shareholders it wants to increase the maximum number of directors on its board from 15 to 18 as it prepares for some directors to retire. According to the company, the move will allow new directors to join the board before others retire and give the new directors time to become familiar with the company, the industry and the way the board works.

Shareholders also approved an advisory “say on pay” vote on executive compensation.

But they voted against a proposal from the Teamsters union urging the board’s compensation committee to require senior executives to retain at least 75 percent of shares until they leave the company or reach retirement age. The Teamsters said the aim would be to “better align” the interests of executives with the interests of shareholders and the company. The board had recommended a vote against the proposal, saying it already has stock ownership guidelines.