U.S. airline shares are off to their worst start since 2011 amid concern that carriers are offering too many seats, chipping away at one of the industry’s most closely watched measures of sales.

Revenue from each seat flown a mile fell in the first quarter and probably will decline again in the coming three months, according to analysts, including Julie Yates of Credit Suisse Group AG and UBS AG’s Darryl Genovesi. Some of those results may show up this week in airline traffic reports.