Behavioural adaptations to flight into thin air

Sherub, Sherub; Bohrer, Gil; Wikelski, Martin; Weinzierl, Rolf

doi:10.1098/rsbl.2016.0432

Cited by 33 publications

(31 citation statements)

References 11 publications

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“…Our finding that birds modulate radius by changing bank angle is in contrast to that of a recent study on Himalayan griffon vultures soaring in excess of 6000 m (Sherub et al, 2016). Although the Himalayan griffons also increased their radius with altitude, they achieved this by increasing their airspeed (keeping bank angle constant).…”

Section: Discussioncontrasting

confidence: 99%

“…Indeed, such angles have been observed from gliders (e.g. Shannon et al, 2002) and in Himalayan vultures (Gyps himalayensis) flying at low altitudes (Sherub et al, 2016). However, the predicted 24 deg is arrived at by assuming that birds are aiming to minimise both their turn radius (and thus remain near the 'core' of the thermal with the strongest uplift) and their sink rate.…”

Section: Introductionmentioning

confidence: 99%

“…Few studies have quantified bank angle directly, although some in-flight angular measurements have previously been recorded incidentally using on-board cameras, for example, to quantify the lateral displacement of the tail in the flight manoeuvres of a steppe eagle, Aquila nipalensis (Gillies et al, 2011). Turning radii can also be derived using GPS data (adjusted for wind drift) or measures of airspeed (Treep et al, 2016;Weinzierl et al, 2016;Horvitz et al, 2014;Sherub et al, 2016). However, deriving bank angle from these measures of turn radius assumes that birds adopt the angles that are required for theoretically ideal circling flight (cf.…”

Section: Introductionmentioning

confidence: 99%

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Vultures respond to challenges of near-ground thermal soaring by varying bank angle

Williams

Duriez

Holton

et al. 2018

Journal of Experimental Biology

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Many large birds rely on thermal soaring flight to travel cross-country. As such, they are under selective pressure to minimise the time spent gaining altitude in thermal updrafts. Birds should be able to maximise their climb rates by maintaining a position close to the thermal core through careful selection of bank angle and airspeed; however, there have been few direct measurements of either parameter. Here, we apply a novel methodology to quantify the bank angles selected by soaring birds using on-board magnetometers. We couple these data with airspeed measurements to parameterise the soaring envelope of two species of Gyps vulture, from which it is possible to predict 'optimal' bank angles. Our results show that these large birds respond to the challenges of gaining altitude in the initial phase of the climb, where thermal updrafts are weak and narrow, by adopting relatively high, and conserved, bank angles (25-35 deg). The bank angle decreased with increasing altitude, in a manner that was broadly consistent with a strategy of maximising the rate of climb. However, the lift coefficients estimated in our study were lower than those predicted by theoretical models and wind-tunnel studies. Overall, our results highlight how the relevant currency for soaring performance changes within individual climbs: when thermal radius is limiting, birds vary bank angle and maintain a constant airspeed, but speed increases later in the climb in order to respond to decreasing air density.

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Section: Discussioncontrasting

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Vultures respond to challenges of near-ground thermal soaring by varying bank angle

Williams

Duriez

Holton

et al. 2018

Journal of Experimental Biology

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“…This research adds to the growing body of work that uses onboard data loggers to test the underlying theory of soaring flight (Shepard, Ross, & Portugal, 2016;Sherub et al, 2016;Treep et al, 2016;Weinzierl et al, 2016). In addition to understanding the costs and benefits of inhabiting contrasting energy landscapes, improved knowledge of flight behaviors can be used for modeling potential anthropogenic risks, in particular the effects of wind turbines and associated energy infrastructure (McLeod, Whitfield, & McGrady, 2002;Reid et al, 2015).…”

Section: Discussionmentioning

confidence: 99%

“…Therefore, the preferential use of areas that maximize net energy gain is expected. Although there is good theoretical understanding of uplift availability and its use for soaring (Bohrer et al, 2012;Pennycuick, 2008), empirical studies using modern technology to investigate whether birds utilize uplift in line with these expectations are only now emerging Lanzone et al, 2012;Péron et al, 2017;Shepard, Lambertucci, Vallmitjana, & Wilson, 2011;Sherub, Bohrer, Wikelski, & Weinzierl, 2016).…”

Section: Introductionmentioning

confidence: 99%

Where eagles soar: Fine‐resolution tracking reveals the spatiotemporal use of differential soaring modes in a large raptor

Murgatroyd

Photopoulou

Underhill

et al. 2018

Ecology and Evolution

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Unlike smaller raptors, which can readily use flapping flight, large raptors are mainly restricted to soaring flight due to energetic constraints. Soaring comprises of two main strategies: thermal and orographic soaring. These soaring strategies are driven by discrete uplift sources determined by the underlying topography and meteorological conditions in an area. High‐resolution GPS tracking of raptor flight allows the identification of these flight strategies and interpretation of the spatiotemporal occurrence of thermal and orographic soaring. In this study, we develop methods to identify soaring flight behaviors from high‐resolution GPS tracking data of Verreaux’s eagle Aquila verreauxii and analyze these data to understand the conditions that promote the use of thermal and orographic soaring. We use these findings to predict the use of soaring flight both spatially (across the landscape) and temporally (throughout the year) in two topographically contrasting regions in South Africa. We found that topography is important in determining the occurrence of soaring flight and that thermal soaring occurs in relatively flat areas which are likely to have good thermal uplift availability. The predicted use of orographic soaring was predominately determined by terrain slope. Contrary to our expectations, the topography and meteorology of eagle territories in the Sandveld promoted the use of soaring flight to a greater extent than in territories in the more mountainous Cederberg region. Spatiotemporal mapping of predicted flight behaviors can broaden our understanding of how large raptors like the Verreaux’s eagle use their habitat and how that links to energetics (as the preferential use of areas that maximize net energy gain is expected), reproductive success, and ultimately population dynamics. Understanding the fine‐scale landscape use and environmental drivers of raptor flight can also help to predict and mitigate potential detrimental effects of anthropogenic developments, such as mortality via collision with wind turbines.

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Wind estimation based on thermal soaring of birds

Weinzierl

Bohrer

Kranstauber

et al. 2016

Ecology and Evolution

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The flight performance of birds is strongly affected by the dynamic state of the atmosphere at the birds' locations. Studies of flight and its impact on the movement ecology of birds must consider the wind to help us understand aerodynamics and bird flight strategies. Here, we introduce a systematic approach to evaluate wind speed and direction from the high‐frequency GPS recordings from bird‐borne tags during thermalling flight. Our method assumes that a fixed horizontal mean wind speed during a short (18 seconds, 19 GPS fixes) flight segment with a constant turn angle along a closed loop, characteristic of thermalling flight, will generate a fixed drift for each consequent location. We use a maximum‐likelihood approach to estimate that drift and to determine the wind and airspeeds at the birds' flight locations. We also provide error estimates for these GPS‐derived wind speed estimates. We validate our approach by comparing its wind estimates with the mid‐resolution weather reanalysis data from ECMWF, and by examining independent wind estimates from pairs of birds in a large dataset of GPS‐tagged migrating storks that were flying in close proximity. Our approach provides accurate and unbiased observations of wind speed and additional detailed information on vertical winds and uplift structure. These precise measurements are otherwise rare and hard to obtain and will broaden our understanding of atmospheric conditions, flight aerodynamics, and bird flight strategies. With an increasing number of GPS‐tracked animals, we may soon be able to use birds to inform us about the atmosphere they are flying through and thus improve future ecological and environmental studies.

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Behavioural adaptations to flight into thin air

Cited by 33 publications

References 11 publications

Vultures respond to challenges of near-ground thermal soaring by varying bank angle

Vultures respond to challenges of near-ground thermal soaring by varying bank angle

Where eagles soar: Fine‐resolution tracking reveals the spatiotemporal use of differential soaring modes in a large raptor

Wind estimation based on thermal soaring of birds

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