An updated estimation of cost of transport for flying species

Flying animals are known to modify their movements to benefit from the energy available in the landscape. However, seminal works on the cost of transport (COT) of flying animals largely ignore the effect that physical environment and variation in flight behaviour have on flight costs, identifying flight as the most costly form of locomotion per unit time. The ability to switch from flapping to passive (soaring/gliding) flight, in response to variations in the wind vectors, was assumed to be the province of selected large birds, but modern bio-logging and radar techniques now allow us to accurately estimate the proportion of flapping vs passive flight.In this study, we used a large dataset of high-resolution movement data from 45 species to provide the first assessment of the role of the physical environment in determining COT in the wild. We quantified how changes in the use of flapping flight and variation in the wind vectors contribute to variation in COT, and show how these affect species differently, according to morphology and body mass. This changes the current dogma about the cost of flight relative to other forms of locomotion, demonstrating that flight can be cheaper than swimming and more costly than running, with the difference being driven by the interaction of energy landscape, body mass and wing morphology.