U.S. airlines are bringing back more pilots as they prepare for an expected travel rebound.

Delta Air Lines Inc. said Monday that it recalled all of its 1,713 idled pilots on April 1, before staffing shortages contributed to about 100 flight cancellations Sunday. About 400 will be trained in time to start flying again this summer, the company said. Southwest Airlines Co., meanwhile, said 209 pilots will return from voluntary leave on June 1.

The pilot recalls show how U.S. airlines are stepping up efforts to prepare for a potentially busy summer as more people receive COVID-19 vaccinations. United Airlines Holdings Inc. said last week it would hire 300 new pilots, underscoring the dramatic reversal for an industry that a year ago was slashing payrolls through buyouts, early retirement and temporary leave programs.

More than 1.5 million passengers passed through domestic security checkpoints April 4, compared with just 122,029 a year earlier. United and American Airlines Group Inc. have said their planes are about 80 per cent full.

Delta’s reliability will improve this spring and summer “as more of our workforce becomes vaccinated and more pilots return to active flying,” the company said in an emailed statement.