o:3664 Small energy benefits of in-wake flying in long-duration migratory flights en During long-distance migrations, some bird species make use of in-wake flying, which should allow them to profit from the upwash produced by another bird. While indirect evidence supports energy saving as the primary benefit of in-wake flying, measurements are still missing. We equipped migrating northern bald ibises (Geronticus eremita) with high-precision global navigation satellite system data loggers to track their position in the flock. We estimated birds' energy expenditure through different proxies, namely dynamic body acceleration (DBA), heart rate and effective wingbeat frequency. During active flapping flight, DBA estimates dropped off when in-wake compared with when not-in-wake. In addition, effective wingbeat frequency decreased, suggesting an increased use of intermittent gliding flight during in-wake periods. Heart rate varied greatly among individuals, with a clear decrease during gliding-corroborating its energy-saving function. Furthermore, we found consistent proof for decreased heart rate during in-wake flying, by up to 4.2%. Hence, we have shown that flying in the wake of another individual reduces birds' DBA, heart rate and effective wingbeat frequency, which could reflect reduced energy requirement. Animals; Flight; Animal Migration; Energy Metabolism; Birds; Heart Rate; Wings; Acceleration 1552099 10.1098/rspb.2024.1173 2024-10-29T09:15:21.475Z 44 yes 46 Elisa Perinot University of Veterinary Medicine Vienna / Waldrappteam 0000-0003-0379-8508 Ortal Mizrahy Rewald University of Veterinary Medicine Vienna / Waldrappteam person 0000-0003-1909-0459 Johannes Fritz University of Vienna / Waldrappteam person 0000-0003-4691-2892 Marco Salvatore Nobile Ca' Foscari University of Venice / University of Milano-Bicocca person 0000-0002-7692-7203 Alexei Vyssotski ETH Zurich person Thomas Ruf University of Veterinary Medicine Vienna person 0000-0002-9235-7079 Leonida Fusani University of Veterinary Medicine Vienna / University of Vienna person 0000-0001-8900-796X Bernhard Voelkl Waldrappteam / University of Bern person 0000-0001-5454-2508 application/pdf 1059390 https://phaidra.vetmeduni.ac.at/o:3664 no yes 16 70 1552253 Proceedings of the Royal Society B - Biological Science 10 291 2030 The Royal Society 2024