How swift are swifts Apus apus?

Swifts Apus apus are renowned for their fast flight manner which has fascinated people in all times. However, previous studies of swifts in flight during migration and roosting flights have shown that the birds operate over a narrow range of flight speeds compared with most other birds studied. In this study we have focused on the special flight behavior often called 'screaming parties'. During these flights the birds appear to reach very high speeds and therefore we used a stereo high speed camera setup to measure the flight speeds of the birds during this behavior with high accuracy. The birds were found to fly at much higher speeds during 'screaming parties' than during migration or roosting, on average twice as fast, 20.9 ms-1 (+/- 5.1 ms-1) in horizontal speed. The highest record was 31.1 ms-1 which is the highest measured yet for a swift in self powered flight. Furthermore, the birds were performing steep climbing flights, on average 4.0 ms-1 (+/- 2.8 ms-1) in vertical velocity. A clear trade-off between horizontal speed and vertical speed was found, suggesting that the birds are operating at their maximum.