Effect of tag attachment on the flight performance of five raptor species

Abstract

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.

Evaluation of tag attachment techniques for plunge‐diving terns

“…Given the relatively short deployment duration of tags and the absence of a survival effect in gluemounted tags, one explanation for the lower apparent survival rates of birds with harnesses is an increased risk of entanglement in the harness. The risk of entanglement in harnesses has been identified on several occasions (Foster et al 1992, Herrod et al 2014, Dixon et al 2016, Longarini et al 2023) and lower survival rates of birds with harnesses due to entanglement have been recorded for seabirds (Raine et al 2011). We recorded three cases of entanglement during our studies with a harness strand being caught in the corner of the beak of the bird, similar to the findings in Roseate Terns (Paton et al 2020) and Sooty Terns Onychoprion fuscatus (C. Feare pers.…”

“…2016, Longarini et al . 2023) and lower survival rates of birds with harnesses due to entanglement have been recorded for seabirds (Raine et al . 2011).…”

“…Wind tunnel experiments show that the placement of the tag on the lower dorsal side leads to lower drag in some species (Bowlin et al 2010, Mizrahy-Rewald et al 2023. Moreover, in several raptor species, flight performance was better for leg-loop over full-body harnesses (Longarini et al 2023). Both would favour the use of leg-loops over full-body harnesses, although in other raptor species return rates were lower for birds with leg-loops compared with those with full-body harnesses (Biles et al 2022).…”

“…In all our cases of entanglement, terns got their bill stuck underneath the loop going from the tag, over the shoulder, to the breast. A solution to overcome this problem is the use of a leg-loop harness (Rappole & Tipton 1991, Longarini et al 2023 data) and entanglement has not been recorded so far, although sample sizes were small in all studies. Furthermore, Clewley et al (2022) suggested that for Black-legged Kittiwakes Rissa tridactyla legloop harnesses also seem to be preferred over body harnesses because of the lower incidence of skin abrasion where the harness was sitting, although their study was based on fairly small sample sizes and short deployment durations.…”

Minimum reporting standards can promote animal welfare and data quality in biologging research

“…Instrument weight and shape, among other characteristics, are critical to report, as they could cause changes in flight behavior, migration distances, and survival. Attachment is also important: backpack attachments have been shown to alter flying behavior more than leg loop attachments, likely because of increased drag [100]. These behavioral adjustments may allow species to compensate for instrumentation without altering reproductive behaviors such as chick provisioning [64,101].…”