Atlanta (AP) — Nearly 7,000 flights worldwide were canceled over the busy Memorial Day holiday weekend in the United States, raising even more concerns about airline performance for the upcoming season summer. About 1,640 flights were canceled on Monday, according to flight-tracking website FlightAware, surpassing the previous day’s total by nearly 200. That followed more than 2,300 cancellations on Friday and another 1,500 on Saturday. Nearly 450 of Sunday’s cancellations were for planes scheduled to fly to or from US cities.

Airlines and tourist destinations are anticipating huge crowds this summer as travel restrictions ease and pandemic fatigue overcomes lingering fear of contracting Covid-19 while travelling. Many forecasters believe the number of travelers will equal or even exceed pre-pandemic levels. However, airlines have thousands fewer employees than in 2019, which has at times been a major contributing factor to widespread flight cancellations.

Delta Air Lines canceled the most flights among major US airlines over the weekend, with more than 250 flights, or 9% of its operations, eliminated on Saturday. One hundred and sixty Delta flights were canceled Sunday evening, according to FlightAware.

Saturday’s cancellations were due to bad weather and “air traffic control actions,” Atlanta-based Delta said in an email to The Associated Press, noting it had tried to cancel flights at least 24 hours before Memorial Day weekend.

Delta announced on its website Thursday that from July 1 to August 7, it will reduce service by about 100 daily departures, primarily in parts of the United States and Latin America that Delta flies frequently.

“More than ever in our history, the various factors that are currently impacting our operations – weather and air traffic control, supplier personnel, rising Covid case rates contributing to higher than expected unplanned absences in certain workgroups — results in an operation that doesn’t always live up to the standards Delta has set for the industry in recent years,” Allison Ausband, Delta’s chief customer experience officer, said in a post.

People who are only now booking trips for the summer are experiencing sticker shock resulting from high demand and limited supply. Domestic airline fares for the summer are averaging more than $400 for a round trip, 24% higher than at this time in 2019, before the pandemic, and 45% higher than a year ago, according to travel data firm Hopper.