Scacco, Martina
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Effect of harness design for tag attachment on the flight performance of five soaring species
Background: Bio-logging devices play a fundamental and indispensable role in movement ecology studies, particularly in the wild. However, researchers are aware of the influence 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 consequences for the collected data, and quantifying the type and magnitude of such potential effects is fundamental to enable researchers to combine and compare data from different studies, as much as it is to improve animal welfare. For 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 different harness types used on these species are scarce. Methods: 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 soaring 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, airspeed, 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. Results: Birds equipped with leg-loops climbed up to 0.36 ms−1 faster, reached 25.9% greater altitudes while soaring and spent less time in active flight compared to birds equipped with backpacks, suggesting that backpack harnesses, compared to leg-loops, might cause additional drag affecting the birds’ flight performance. A lower VeDBA, a lower rate of sinking while gliding and slightly higher glide ratio and airspeeds were also indicative of less drag using leg-loops, even though the effect on these parameters was comparable to inter-individual differences. Conclusions: Our results add to the existing literature highlighting the design-related advantages of leg-loops, and support the use of leg-loops as a better alternative to backpack harnesses for large soaring birds, when possible. Our study also highlights how apparently small changes in device attachment can lead to notable improvements in tagging practice, with implications for animal welfare, data interpretation and comparability.
Wing tags severely impair movement in African Cape Vultures
Background
The use of tracking technologies is key for the study of animal movement and pivotal to ecological and conservation research. However, the potential effects of devices attached to animals are sometimes neglected. The impact of tagging not only rises welfare concerns, but can also bias the data collected, causing misinterpretation of the observed behaviour which invalidates the comparability of information across individuals and populations. Patagial (wing) tags have been extensively used as a marking method for visual resightings in endangered vulture species, but their effect on the aerodynamics of the birds and their flight behaviour is yet to be investigated. Using GPS backpack mounted devices, we compared the flight performance of 27 captive and wild Cape Vultures (Gyps coprotheres), marked with either patagial tags or coloured leg bands.
Results
Individuals equipped with patagial tags were less likely to fly, travelled shorter distances and flew slower compared to individuals equipped with leg bands. These effects were also observed in one individual that recovered its flight performance after replacing its patagial tag by a leg band.
Conclusions
Although we did not measure the effects of patagial tags on body condition or survival, our study strongly suggests that they have severe adverse effects on vultures’ flight behaviour and emphasises the importance of investigating the effects that tagging methods can have on the behaviour and conservation of the study species, as well as on the quality of the scientific results.
The species-specificity of energy landscapes for soaring birds, and its consequences for transferring suitability models across species
Context
Soaring birds depend on atmospheric uplifts and are sensitive to wind energy development. Predictive modelling is instrumental to forecast conflicts between human infrastructures and single species of concern. However, as multiple species often coexist in the same area, we need to overcome the limitations of single species approaches.
Objectives
We investigate whether predictive models of flight behaviour can be transferred across species boundaries.
Methods
We analysed movement data from 57 white storks, Ciconia ciconia, and 27 griffon vultures, Gyps fulvus. We quantified the accuracy of topographic features, correlates of collision risk in soaring birds, in predicting their soaring behaviour, and tested the transferability of the resulting suitability models across species.
Results
59.9% of the total area was predicted to be suitable to vultures only, and 1.2% exclusively to storks. Only 20.5% of the study area was suitable to both species to soar, suggesting the existence of species-specific requirements in the use of the landscape for soaring. Topography alone could accurately predict 75% of the soaring opportunities available to storks across Europe, but was less efficient for vultures (63%). While storks relied on uplift occurrence, vultures relied on uplift quality, needing stronger uplifts to support their higher body mass and wing loading.
Conclusions
Energy landscapes are species-specific and more knowledge is required to accurately predict the behaviour of highly specialised soaring species, such as vultures. Our models provide a base to explore the effects of landscape changes on the flight behaviour of different soaring species. Our results suggest that there is no reliable and responsible way to shortcut risk assessment in areas where multiple species might be at risk by anthropogenic structures.
Bats use topography and nocturnal updrafts to fly high and fast
During the day, flying animals exploit the environmental energy landscape by seeking out thermal or orographic uplift, or extracting energy from wind gradients.1-6 However, most of these energy sources are not thought to be available at night because of the lower thermal potential in the nocturnal atmosphere, as well as the difficulty of locating features that generate uplift. Despite this, several bat species have been observed hundreds to thousands of meters above the ground.7-9 Individuals make repeated, energetically costly high-altitude ascents,10-13 and others fly at some of the fastest speeds observed for powered vertebrate flight.14 We hypothesized that bats use orographic uplift to reach high altitudes,9,15-17 and that both this uplift and bat high-altitude ascents would be highly predictable.18 By superimposing detailed three-dimensional GPS tracking of European free-tailed bats (Tadarida teniotis) on high-resolution regional wind data, we show that bats do indeed use the energy of orographic uplift to climb to over 1,600 m, and also that they reach maximum sustained self-powered airspeeds of 135 km h-1. We show that wind and topography can predict areas of the landscape able to support high-altitude ascents, and that bats use these locations to reach high altitudes while reducing airspeeds. Bats then integrate wind conditions to guide high-altitude ascents, deftly exploiting vertical wind energy in the nocturnal landscape.
Environmental and social correlates, and energetic consequences of fitness maximisation on different migratory behaviours in a long-lived scavenger
Partial migration is one of the most widespread migratory strategies among taxa. Investigating the trade-off between envi- ronmental/social factors — fitness and energetic consequences — is essential to understand the coexistence of migratory and resident behaviours. Here, we compiled field monitoring data of wintering population size and telemetry data of 25 migrant and 14 resident Egyptian Vultures Neophron percnopterus to analyse how environmental and social factors modu- late overwintering immature population size, compare energetic consequences between migratory and resident individuals across wintering and non-wintering seasons and evaluate fitness components (i.e. survival and reproduction) between the two migratory forms. We observed that social attraction may influence the number of overwintering immature individuals, which increased linearly with adult birds surveyed. Residents spent more energy but exhibited higher survival probabilities and lower breeding activity. On the contrary, migratory birds showed lower energy expenditure during winter but also lower survival and more breeding attempts. These results suggest that social attraction may modulate population dynamics and promote residency in immature birds. Resident individuals benefit from enhancing their survival at the expense of higher energy expenditure during winter. Migrant birds, on the contrary, may compensate for the higher costs in terms of survival by a reduction in the energy cost, which may benefit more frequent breeding. Our results offer new insights to understand how species benefit from one strategy or another and that the coexistence of both migratory forms is context-dependent.
Static landscape features predict uplift locations for soaring birds across Europe
Soaring flight is a remarkable adaptation to reduce movement costs by taking advantage of atmospheric uplifts. The movement pattern of soaring birds is shaped by the spatial and temporal availability and intensity of uplifts, which result from an interaction of local weather conditions with the underlying landscape structure. We used soaring flight locations and vertical speeds of an obligate soaring species, the white stork (Ciconia ciconia), as proxies for uplift availability and intensity. We then tested if static landscape features such as topography and land cover, instead of the commonly used weather information, could predict and map the occurrence and intensity of uplifts across Europe. We found that storks encountering fewer uplifts along their routes, as determined by static landscape features, suffered higher energy expenditures, approximated by their overall body dynamic acceleration. This result validates the use of static features as uplift predictors and suggests the existence of a direct link between energy expenditure and static landscape structure, thus far largely unquantified for flying animals. Our uplift availability map represents a computationally efficient proxy of the distribution of movement costs for soaring birds across the world's landscapes. It thus provides a base to explore the effects of changes in the landscape structure on the energy expenditure of soaring birds, identify low-cost movement corridors and ultimately inform the planning of anthropogenic developments.