Effect of tag attachment on flight parameters and energetic cost across five raptor species

Bio-logging devices play a fundamental and indispensable role in movement ecology studies, particularly in the wild. However, researchers are becoming increasingly aware of the potential effects that attaching devices can have on animals, particularly on their behaviour, energy expenditure and survival. The way a device is attached to an animal's body has also potential effects on the collected data, and quantifying the type and magnitude of potential bias is fundamental to enable researchers to combine and compare data from different studies. Since over two decades, large terrestrial birds have been in the focus of long-term movement ecology research, employing bio-logging devices attached with different types of harnesses. However, comparative studies investigating the effects of harness type on these species are scarce. In this study, we tested for potential differences in data collected by two commonly used harness types, backpack and leg-loop, on the flight performance of 10 individuals from five raptor species, equipped with high resolution bio-logging devices, in the same area and time. We explored the effect of harness type on vertical speed, horizontal speed, glide ratio, height above sea level, distance travelled, proportion of soaring and flapping behaviour, and VeDBA (a proxy for energy expenditure) between and within individuals, all used as fine-scale measures of flight performance. Birds equipped with leg-loops climbed up to 0.65 ms-1 faster, reached 19% greater heights while soaring and spent less time with active flight compared to birds equipped with backpacks, suggesting that backpack harnesses, compared to leg-loops, might cause additional drag that lowered the birds' flight performance. A lower rate of sinking while gliding, a slightly higher glide ratio, higher horizontal speed while soaring, and lower VeDBA, were also indicative of decreased drag using leg-loops. Our results, add to the pre-existing literature highlighting the design-related advantages of leg-loops, and they are in support of considering leg-loops as a better alternative to backpack harnesses for large soaring birds. Our study also highlights the importance of investigating how the methodology used to measure behavioural information affects the collected data to avoid systematic bias, which would invalidate data comparability and lead to misinterpreting the behaviour being measured.