Yes, they only come here to breed. Extraordinary birds...
"Youngsters are independent as soon as they leave the nest, and set out immediately on migration.
Swifts start their return journey in mid July, before nights become too cool. They canít roost overnight during the journey, like swallows do, so they travel quickly. One young swift that left its UK nest on 31 July, was found in Madrid (Spain) on 3 August."