Delta Air Lines Inc. and American Airlines Group Inc. are cutting regional flights over the next few months due to pilot shortages, extending a lingering effect from the early days of the coronavirus pandemic.

Delta has cut back by as much as 25% for the first half of this year, the Atlanta-based carrier said on an earnings call Thursday. American has dropped 580 March flights at its wholly owned Piedmont Airlines and an undisclosed number this month and next at regional partners SkyWest Inc. and Mesa Air Group Inc.

When airlines moved to replace thousands of pilots who took incentives to retire early in 2020, the carriers turned first to their traditional hiring pool at regional partners. Pandemic-related disruptions to training at those smaller airlines contributed to the shortage by preventing first officers from being promoted to captain.

Delta expects to add 100 to 200 pilots a month into next year, Chief Executive Officer Ed Bastian said on the earnings call. The company hasn’t had a shortage of pilots or applicants at its mainline operations, he said.

The airline is “pretty confident” that by the second half of this year, it will be able to restore services to communities that were cut in the first half, President Glen Hauenstein said on the call.

American is proactively adjusting its schedule “to mitigate any future travel disruptions” from near-term pilot shortages at the regional carriers, including those ill from the omicron coronavirus variant, the airline said in a statement. It has not left any markets, it said. SkyWest and Mesa didn’t immediately respond to requests for comment.

Delta’s mainline operation hasn’t exited any cities, although its regional partners, which ferry passengers from midsize and small cities to major hubs, have left “a handful of markets,” Bastian said. Airline Weekly reported last month that Delta had left three locations and was suspending another 10 routes.

Rules blocking airlines from ending flights to cities under federal aid provisions have expired, Delta said.

United Airlines Holdings Inc. has grounded 100 regional jets because of the same problem, CEO Scott Kirby said in December, and it has discontinued service to an unspecified number of cities.

Delta shares rose 2.1% Thursday in New York, while United climbed 3.5% and American jumped 4.5%.