No one knew this until Lund University in Sweden attached geolocator tracking devices to common swifts in 2015. When the swifts returned from their winter range in Africa the Swedes recaptured the tagged birds and found that most had not landed since they left!
Since common swifts spend so much time in the air and can live for 20 years an individual swift may fly 3 million miles in his lifetime. Wow.
(image from Wikimedia Commons; click on the caption to see the original)