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"Soaring"
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Glider soaring via reinforcement learning in the field
Soaring birds often rely on ascending thermal plumes (thermals) in the atmosphere as they search for prey or migrate across large distances
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. The landscape of convective currents is rugged and shifts on timescales of a few minutes as thermals constantly form, disintegrate or are transported away by the wind
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. How soaring birds find and navigate thermals within this complex landscape is unknown. Reinforcement learning
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provides an appropriate framework in which to identify an effective navigational strategy as a sequence of decisions made in response to environmental cues. Here we use reinforcement learning to train a glider in the field to navigate atmospheric thermals autonomously. We equipped a glider of two-metre wingspan with a flight controller that precisely controlled the bank angle and pitch, modulating these at intervals with the aim of gaining as much lift as possible. A navigational strategy was determined solely from the glider’s pooled experiences, collected over several days in the field. The strategy relies on on-board methods to accurately estimate the local vertical wind accelerations and the roll-wise torques on the glider, which serve as navigational cues. We establish the validity of our learned flight policy through field experiments, numerical simulations and estimates of the noise in measurements caused by atmospheric turbulence. Our results highlight the role of vertical wind accelerations and roll-wise torques as effective mechanosensory cues for soaring birds and provide a navigational strategy that is directly applicable to the development of autonomous soaring vehicles.
A reinforcement learning approach allows a suitably equipped glider to navigate thermal plumes autonomously in an open field.
Journal Article
Skybound : a journey in flight
In her mid-30s Rebecca Loncraine was diagnosed with breast cancer. Two years later, and after months of gruelling treatment, she flew in a glider for the first time. In that engineless plane, soaring 3000 feet over the landscape of her childhood with only the rising thermals to take her higher and the birds to lead the way, she fell in love. If illness meant Rebecca had lost touch with the world around her, gliding showed her a way to learn to live again. And so Rebecca travelled from the Black Mountains in Wales to New Zealand's Southern Alps and the Nepalese Himalayas to chase her newfound passion: her need to fly with the birds, to push herself to the boundary of her own fear. Skybound is the story of that obsession and of Rebecca's incredible journey from the ground, into the sky and back again. Taking in the history of unpowered flight, and with extraordinary descriptions of flying in some of the world's most dangerous and dramatic locations, this is a nature memoir with a unique perspective; it is about the land we know and the sky we know so little of, it is about memory and self-discovery. Just as she finished writing Skybound Rebecca became ill again. She died in September 2016. And yet, Skybound is still a book about learning to live again: deeply moving, thrilling and euphoric, this is a book for anyone who has ever looked up and wanted to take flight.
Fine‐Scale Movement Data Reveal Primarily Surface Foraging and Nocturnal Flight Activity in the Endangered Bermuda Petrel
Foraging behaviour plays a fundamental role in animal fitness and population dynamics., particularly for central‐place foragers like breeding seabirds. Among Procellariiform seabirds, petrels exhibit a wide range of foraging strategies finely tuned to the patchy and unpredictable distribution of resources. The extent and remote nature of their foraging grounds makes direct observation of foraging behaviour impractical, thereby requiring the use of remote tracking technologies. We deployed miniaturised multi‐sensor biologgers and collected fine scale movement data to investigate the at‐sea behaviours of the Bermuda petrel Pterodroma cahow, a poorly studied and highly threatened gadfly petrel, specialised on mesopelagic prey. GPS‐tracking data revealed extensive foraging trips (mean ± SD: 1207 ± 305 km), in consistent directions, over remote oceanic regions. Time‐depth‐recorders provided new insights into Bermuda petrel feeding techniques suggesting that the meso‐bathypelagic prey targeted by petrels must be available in the very upper layer of the water surface, given their very limited diving activity (maximum dive depth of 1.57 m). We identified three flight‐related and three water‐associated behaviours using supervised classification approach to classify behaviour from tri‐axial acceleromtetry. Flying behaviours reflected the expected dynamic soaring flight strategy of Procellariiformes; individuals spent more than 75% of their time in flight (dynamic soaring and flap‐gliding) with dynamic soaring flight being the most common behaviour under all conditions. The behaviour classified as ‘Intensive flight’ was infrequently observed but could indicate aerial dipping, a characteristic foraging technique of Pterodroma species. The remaining time was spent in three water behaviours: active, inactive and intensive, with the latter being less common but thought to reflect scavenging and prey seizing. Flight increased during dusk and in the night, highlighting greater flight activity during night compared to the day, while water behaviours were more common during the day. While some of our findings may require further validation to confirm their relevance to foraging behaviour, our work offers new and valuable insights to consider when assessing the ecological needs of this endangered species and its potential vulnerability to offshore anthropogenic activities.
Journal Article
Optimization of Thrust-Augmented Dynamic Soaring
Dynamic soaring is a non-powered flight mode that enables to fly at no cost by gaining energy from a horizontal shear wind. This is not possible if the shear wind strength is too low. Engine thrust is introduced as a means to augment dynamic soaring in shear winds with insufficient strength. Appropriate models of the vehicle dynamics and the shear wind are developed, and an optimization method is used to construct results on optimal thrust-augmented dynamic soaring. The minimum energy required from the propulsion system is determined for the entire region of insufficient shear wind strength down to zero wind. Solutions of the characteristics of the motion including states and controls are presented. Furthermore, it is shown what a control simplification in terms of an optimal constant power setting as an ease of control yields for the energy required from the propulsion system.
Journal Article
The gateway to Africa
Large bodies of water represent major obstacles for the migration of soaring birds because thermal updrafts are absent or weak over water. Soaring birds are known to time their water crossings with favourable weather conditions and there are records of birds falling into the water and drowning in large numbers. However, it is still unclear how environmental factors, individual traits and trajectory choices affect their water crossing performance, this being important to understand the fitness consequences of water barriers for this group of birds. We addressed this problem using the black kite Milvus migrans as model species at a major migration bottleneck, the Strait of Gibraltar. We recorded high‐resolution GPS and triaxial accelerometer data for 73 birds while crossing the Strait of Gibraltar, allowing the determination of sea crossing duration, length, altitude, speed and tortuosity, the flapping behaviour of birds and their failed crossing attempts. These parameters were modelled against wind speed and direction, time of the day, solar irradiance (proxy of thermal uplift), starting altitude and distance to Morocco, and age and sex of birds. We found that sea crossing performance of black kites is driven by their age, the wind conditions, the starting altitude and distance to Morocco. Young birds made longer sea crossings and reached lower altitude above the sea than adults. Crosswinds promoted longer sea crossings, with birds reaching lower altitudes and with higher flapping effort. Birds starting at lower altitudes were more likely to quit or made higher flapping effort to complete the crossing. The location where birds started the sea crossings impacted crossing distance and duration. We present evidence that explains why migrating soaring birds accumulate at sea passages during adverse weather conditions. Strong crosswinds during sea crossings force birds to extended flap‐powered flight at low altitude, which may increase their chances of falling in the water. We also showed that juvenile birds assume more risks than adults. Finally, the way in which birds start the sea crossing is crucial for their success, particularly the starting altitude, which dictates how far birds can reach with reduced flapping effort. Soaring birds engage in flap‐powered flight during sea crossings, which is fatiguing and can lead to drowning due to exhaustion. Using state‐of‐art tracking technology, the authors determined precise indicators of sea crossing performance. They found that sea crossing performance is driven by age, wind conditions and the starting trajectory.
Journal Article
Physical limits of flight performance in the heaviest soaring bird
Flight costs are predicted to vary with environmental conditions, and this should ultimately determine the movement capacity and distributions of large soaring birds. Despite this, little is known about how flight effort varies with environmental parameters. We deployed bio-logging devices on the world’s heaviest soaring bird, the Andean condor (Vultur gryphus), to assess the extent to which these birds can operate without resorting to powered flight. Our records of individual wingbeats in >216 h of flight show that condors can sustain soaring across a wide range of wind and thermal conditions, flapping for only 1% of their flight time. This is among the very lowest estimated movement costs in vertebrates. One bird even flew for >5 h without flapping, covering ∼172 km. Overall, > 75% of flapping flight was associated with takeoffs. Movement between weak thermal updrafts at the start of the day also imposed a metabolic cost, with birds flapping toward the end of glides to reach ephemeral thermal updrafts. Nonetheless, the investment required was still remarkably low, and even in winter conditions with weak thermals, condors are only predicted to flap for ∼2 s per kilometer. Therefore, the overall flight effort in the largest soaring birds appears to be constrained by the requirements for takeoff.
Journal Article
The characteristic time-scale of perceived information for decision-making
Animals are often required to make decisions about their use of current resources while minimising travel costs and risks due to uncertainty about the forthcoming resources. Passive soaring birds utilise warm rising‐air columns (thermals) to climb up and obtain potential energy for flying across large areas. However, the utilisation of such inconsistent natural resources may be challenging for soaring‐gliding birds and involve a set of decisions to maintain efficient flight. To assess which temporal scales of previous experience with environmental inputs best predicted thermal‐climbing departure decisions of soaring birds, we used movement data from Eurasian griffon vultures (Gyps fulvus) tracked by GPS transmitters. We applied Cox proportional hazard regression and a model selection approach to identify thermal‐climbing departure decisions and to compare a range of temporal scales. Our findings support the use of current and recent (short‐term; last 20 min) experiences, compared to longer term, past experiences, in predicting the time until departure from thermals. The models supported decision rules that integrated information originating from different temporal scales, implying a tendency to depart from a thermal later when the current climb rate was higher than experienced recently and vice versa. In addition, climb rates in thermals revealed significant autocorrelation over short time‐scales (shorter than 30 min). The correspondence between thermals' characteristics and the factors that best predicted thermal‐climbing departure decisions presumably reflects optimal decisions individuals make to handle their dynamic environment and to reduce movement‐related costs of such a basic activity for soaring‐gliding birds. A plain language summary is available for this article. Plain Language Summary
Journal Article
An Autonomous Soaring for Small Drones Using the Extended Kalman Filter Thermal Updraft Center Prediction Method Based on Ordinary Least Squares
Many birds in the natural world are capable of engaging in sustained soaring within thermal updrafts for extended periods without flapping their wings. Autonomous soaring has the potential to greatly improve both the range and endurance of small drones. In this paper, the extended Kalman filter (EKF) thermal updraft center prediction method based on ordinary least squares (OLS) is proposed to develop the autonomous soaring system for small drones, and an adaptive step size update strategy is incorporated into the EKF. The proposed method is compared with EKF thermal updraft prediction methods through simulated experiments. The results indicate that the proposed prediction method has low computational complexity and fast convergence speed and performs more stably in weak thermal updrafts. The above advantages stem from the OLS providing an approximate distribution of the thermal updraft around the drone for the EKF. This empowers the EKF algorithm with ample information to dynamically update the thermal updraft center in real time. The adaptive step size update strategy further accelerates the convergence speed of this process. In addition, flight experiments were conducted on the Talon fixed-wing drone platform to test the autonomous soaring system. During the flight experiment, the drone successfully engaged in static soaring within thermal updrafts, effectively hovering and gaining energy. Throughout the approximately 40 min flight duration, the drone only utilized its propulsion for about 8 min. This demonstrated the effectiveness of the autonomous soaring system using the EKF thermal updraft center prediction method based on OLS. Finally, by analyzing and discussing the differences between the simulation experiment results and the flight experiment results, some improvement strategies for the current work are proposed.
Journal Article