Person: Scacco, Martina
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Small-scale migratory behavior of three facultative soaring raptors approaching a water body : a radar study investigating the effect of weather, topography and flock size
Water bodies are considered a barrier to the migration of large bird species, mainly because of the absence of thermals that these birds heavily rely on to move large distances with little energy expenditure. In this two-year study, we combined vertical and horizontal radar data with visual observations to compare the autumn migratory behavior of three facultative soaring species: European honey buzzards Pernis apivorus, western marsh harriers Circus aeruginosus and black kites Milvus migrans. Here we used non-parametric tests, linear and generalized linear models to investigate the effect of flock size, age, local weather conditions, time of the day and topography on the small-scale flight behavior of these species, quantified in terms of flight altitude, flight direction and distance from the mountain ridge. European honey buzzards, both adults and juveniles, were detected over the plateau near the mountain chain during suitable weather conditions for soaring flight (especially high temperature) and during high species flow, which facilitated the location of thermals. In contrast, inexperienced juveniles were less concentrated in space, forming smaller flocks and flying at lower altitudes, probably being less facilitated than adult in exploiting the soaring flight. The Western marsh harrier, a raptor largely using the flapping flight even over land, flew lower than adult honey buzzards and nearer to the mountain ridge during strong tailwinds, perhaps being efficient in exploiting their support using the flapping flight even during inter-thermal gliding. Such as western marsh harriers, black kites flew nearer the mountain chain during strong tailwinds, but they probably use soaring flight during such weather conditions to exploit their onward support even when circling in thermals.
Time series enable the characterization of small-scale vegetation dynamics that influence fine-scale animal behavior : an example from white storks' foraging behavior
Agricultural activities and vegetation growth cause rapid small-scale vegetation changes which dynamically alter habitat suitability. Time series enable to track down such variations of vegetation structure and are promising to examine their impact on animals' life. Nevertheless, their potential to characterize vegetation dynamics in ways pertinent to animals' fine-scale habitat use has not been adequately explored and ecologically meaningful proxies are lacking. To address this gap, we exemplary investigated foraging activities of breeding white storks in an agricultural landscape. Reflecting on the understanding that storks require short vegetation to access prey, we examined if good foraging conditions – early growth and post-harvest/mowing periods – are detectable using the points between local minima/maxima in NDVI profiles (half-maximum). We processed 1 year of Landsat imagery to identify half-maximum periods (HM: good prey access) and non-half-maximum periods (non-HM: poor prey access) on field-scale in croplands and grasslands. Additionally, we mapped used/unused fields and retrieved foraging duration/daily visitation rates from GPS tracks of the storks. We then explored habitat use, compared habitat use with habitat availability and tested temporal predictors distinguishing between HM/non-HM in habitat selection models. Examining habitat use, storks revisited croplands and grasslands significantly more often during HM than during non-HM, while foraging duration was only prolonged in croplands during HM. However, comparing habitat use with habitat availability, we observed that storks used croplands and grasslands in significantly higher proportions during HM than during non-HM. Additionally, we found that temporal information affected storks' habitat selection and improved model performance. Our findings emphasize that the half-maximum proxy enables to coarsely distinguish temporal resource variations in storks' foraging habitats, highlighting the potential of time series for characterizing behaviorally-relevant vegetation dynamics. Such information helps to create more species-centered landscape scenarios in habitat models, allowing to unravel effects of small-scale environmental changes on wildlife to ultimately guide conservation and management.
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.
Hypatia-trackRadar : A software for animal tracking using marine surveillance radars
Hypatia-trackRadar is a Java standalone application designed to help biologists extract and process bird movement data from marine surveillance radars. This application integrates simultaneous collection of radar data and field observations by allowing the user to link information gathered from visual observers (such as bird species and flock size) to the radar echoes. A virtual transparent sheet positioned on the radar screen allows the user to visually follow and track the echoes on the radar screen. The application translates the position of the echoes on the screen in a metric coordinate system. Based on time and spatial position of the echoes the software automatically calculates multiple flight parameters, such as ground speed, track length and duration. We validated Hypatia-trackRadar using an unmanned aerial vehicle. Here we present the features of this application software and its first use in a real case study in a raptor migration bottle-neck.
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.
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.
On the role of topography and atmospheric conditions to support efficient movement : How flying animals use the energy in the landscape to travel efficiently
Movement is a scale-dependent process which permeates all aspects of life on Earth. Depending on the scale at which we observe movement in its external context, we will notice that this can be facilitated or constrained by different factors, which in the case of humans and animals, translates to a change of the energetic cost required to move between two locations. During my PhD I investigated factors in the physical environments that can define the energetic cost of movement through the landscape based on previously collected bio-logging data. I focused on flying animals, as the three-dimensional aerial environment in which they move is complex and extremely dynamic, constantly providing animals moving in this medium with challenges and opportunities to remain aloft. The advances in bio-logging devices currently allow us to record animal movement at extremely high spatial and temporal resolution. However, the comparability of data collected with different methodologies is rarely investigated. I therefore focused the first chapter of my thesis on testing the comparability of movement data collected through two common types of device attachments, to pool data from different studies. I dedicated the other three chapters of my thesis to investigate the proportion in which topography and atmospheric conditions contribute to creating potential energy in the environment, allowing flying animals across taxa to travel with low energetic cost. I first focused on soaring birds, which strongly rely on environmental support - in terms of vertical air currents - to move across the landscape. I finally applied a similar methodology and research questions to bats, a completely different study system in which the role of environmental support during flight had been largely overlooked. My PhD work suggests that both weather and static landscape features contribute to creating energy in the landscape. It hints to the existence of converging patterns in the way taxonomically distant species use the potential energy available in the landscape to fly efficiently. At the same time, it highlights the species-specificity of energy landscapes, according to which the same landscape provides different species with different low-cost flight opportunities, depending on their behaviour, morphology and on their ability to interpret the landscape. From a methodological and applied perspective, this translates to the need of different combination of environmental parameters to predict the flight behaviour of different species, with clear consequences for their conservation. Finally, my results show that the different contributions of weather versus static landscape features are also a consequence of the spatial and temporal resolution at which these environmental information are available. This highlights the scale-dependent nature of the movement process and the importance of matching the scale and resolution at which the movement phenomenon and its environmental context are observed and investigated.
Individual tracking reveals long-distance flight-path control in a nocturnally migrating moth
Each year, trillions of insects make long-range seasonal migrations. These movements are relatively well understood at a population level, but how individual insects achieve them remains elusive. Behavioral responses to conditions en route are little studied, primarily owing to the challenges of tracking individual insects. Using a light aircraft and individual radio tracking, we show that nocturnally migrating death's-head hawkmoths maintain control of their flight trajectories over long distances. The moths did not just fly with favorable tailwinds; during a given night, they also adjusted for head and crosswinds to precisely hold course. This behavior indicates that the moths use a sophisticated internal compass to maintain seasonally beneficial migratory trajectories independent of wind conditions, illuminating how insects traverse long distances to take advantage of seasonal resources.
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.
Environmental effects on flying migrants revealed by radar
Migratory animals are affected by various factors during their journeys, and the study of animal movement by radars has been instrumental in revealing key influences of the environment on flying migrants. Radars enable the simultaneous tracking of many individuals of almost all sizes within the radar range during day and night, and under low visibility conditions. We review how atmospheric conditions, geographic features and human development affect the behavior of migrating insects and birds as recorded by radars. We focus on flight initiation and termination, as well as in‐flight behavior that includes changes in animal flight direction, speed and altitude. We have identified several similarities and differences in the behavioral responses of aerial migrants including an overlooked similarity in the use of thermal updrafts by very small (e.g. aphids) and very large (e.g. vultures) migrants. We propose that many aerial migrants modulate their migratory flights in relation to the interaction between atmospheric conditions and geographic features. For example, aerial migrants that encounter crosswind may terminate their flight or continue their migration and may also drift or compensate for lateral displacement depending on their position (over land, near the coast or over sea). We propose several promising directions for future research, including the development and application of algorithms for tracking insects, bats and large aggregations of animals using weather radars. Additionally, an important contribution will be the spatial expansion of aeroecological radar studies to Africa, most of Asia and South America where no such studies have been undertaken. Quantifying the role of migrants in ecosystems and specifically estimating the number of departing birds from stopover sites using low‐elevation radar scans is important for quantifying migrant–habitat relationships. This information, together with estimates of population demographics and migrant abundance, can help resolve the long‐term dynamics of migrant populations facing large‐scale environmental changes.