No effect of carrying a leg-loop harness mounted radio transmitter on flight energy expenditure of a small migratory songbird

Radio transmitters and other miniature electronic devices have greatly enhanced our understanding of avian ecology. However, from both animal welfare and data integrity perspectives, it is crucial to determine if tracking devices adversely affect birds. One important knowledge gap is understanding if carrying tracking devices affects energy costs of flight. We carried out a wind tunnel experiment with a paired design where Yellow-rumped Warblers (<em>Setophaga coronata</em>) flew for two hours with and without very high frequency (VHF) radio transmitters mounted with a leg-loop harness (~3% of body mass). We calculated energy expenditure (power; W) by measuring loss of fat and lean mass during flight with quantitative magnetic resonance. There was no effect of VHF radio transmitters on energy expenditure, indicating small songbirds can carry appropriately sized and mounted devices without experiencing increased flight costs. According to aerodynamic theory, smaller birds should have relatively larger power margins compared to larger birds, enabling them to carry relatively greater payloads. Future studies should examine how VHF radio transmitters and other devices of different masses, shapes, and mounting techniques affect flight ability and energy costs across bird species varying in body size and wing morphology.