application/pdf Proceedings of the Royal Society B - Biological Science 291(2030) (2024) The Royal Society CC BY 4.0 International http://creativecommons.org/licenses/by/4.0/ 2024 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. Perinot, Elisa (University of Veterinary Medicine Vienna / Waldrappteam) Voelkl, Bernhard (Waldrappteam / University of Bern) Fusani, Leonida (University of Veterinary Medicine Vienna / University of Vienna) Ruf, Thomas (University of Veterinary Medicine Vienna) Vyssotski, Alexei (ETH Zurich) Nobile, Marco Salvatore (Ca' Foscari University of Venice / University of Milano-Bicocca) Rewald, Ortal Mizrahy (University of Veterinary Medicine Vienna / Waldrappteam) Fritz, Johannes (University of Vienna / Waldrappteam) doi:10.1098/rspb.2024.1173 https://phaidra.vetmeduni.ac.at/o:3664 eng Animals; Flight; Animal Migration; Energy Metabolism; Birds; Heart Rate; Wings; Acceleration Small energy benefits of in-wake flying in long-duration migratory flights article