The driver accelerated the truck to a speed of 6.7 m s−1 before reaching the 210 m start line and then maintained a speed of 6.765 ± 0.002 m s−1 until 8 m from the experimental enclosure, at which point the driver braked to stop at least 2 m from the front of the enclosure (see Appendix Fig. 1b). A High Frequency Analyzer (HFW59D, Gigahertz solutions, Fürth, Germany) was monitored by the observer behind the screen during the approach to ensure that the radar was functioning properly. If the radar turned off or stopped working before the birds completed their escape flights, that trial was not used.