WestJet led the list of North American carriers in on-time performance (OTP) in May, according to research from Cirium, an aviation analytics company. The airline’s OTP of 85.9% represented a 3.1 percentage-point jump over April.

The overall average for May OTP for North American carriers increased by 1.3 percentage points month over month to 79.3%; however, performance of five of the 10 reporting airlines declined.

Despite holding its second-place ranking, Delta Air Lines’ May OTP slipped 1.5 percentage points to 82.7%, while third-place Alaska Airlines had a 3.2 percentage-point decline to 82.3%. Southwest Airlines experienced the steepest decline —  5.2 percentage points — to 70.4%, placing it last. The carrier with the most-improved average was JetBlue, up 5.5 percentage points to 82%, putting it in fourth place, up from sixth in April.

The remaining North American listees were: American Airlines (79.9%), Air Canada (78.7%), Frontier Airlines (77%), Allegiant (73.6%) and Southwest Airlines (70.4%).

WestJet was the only North American carrier to make Cirium’s list of global on-time airlines at seventh. Saudia topped that list at 90.1%.

JetBlue had May’s highest completion factor at 99.8%, followed by United at 99.4% and Alaska at 99.2%. The number of North American flights canceled in May was 10,309,  compared with 7,718 in April.