Differential Use of Thermal Convection by Soaring Birds Over Central Israel

Shamoun-Baranes, Judy; Leshem, Yossi; Yom‐Tov, Yoram; Liechti, Olivier

doi:10.1093/condor/105.2.208

Cited by 56 publications

(74 citation statements)

References 8 publications

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“…For example, optimal theory assumes that birds enter and exit a climb– glide cycle at the same altitude. However, by exploiting the best thermals (Leshem and Yom‐Tov , Pennycuick ) and the strongest lift within those thermals (Shamoun‐Baranes et al ) they can achieve greater migration speed in other ways than by gliding at V opt . Migrant birds can also reduce travel time by adjusting orientation behaviour in relation to wind (Alerstam , McLaren et al ).…”

Section: Discussionmentioning

confidence: 99%

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Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Vansteelant

Shamoun‐Baranes

McLaren

et al. 2017

Journal of Avian Biology

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Thermal soaring birds reduce flight-energy costs by alternatingly gaining altitude in thermals and gliding across the earth's surface. To find out how soaring migrants adjust their flight behaviour to dynamic atmospheric conditions across entire migration routes, we combined optimal soaring migration theory with high-resolution GPS tracking data of migrating honey buzzards Pernis apivorus and wind data from a global numerical atmospheric model.We compared measurements of gliding air speeds to predictions based on two distinct behavioural benchmarks for thermal soaring flight. The first being a time-optimal strategy whereby birds alter their gliding air speeds as a function of climb rates to maximize cross-country air speed over a full climb-glide cycle (V opt ). The second a risk-averse energy-efficient strategy at which birds alter their gliding air speed in response to tailwinds/headwinds to maximize the distance travelled in the intended direction during each glide phase (V bgw ).Honey buzzards were gliding on average 2.05 ms -1 slower than V opt and 3.42 ms -1 faster than V bgw while they increased air speeds with climb rates and reduced air speeds in tailwinds. They adopted flexible flight strategies gliding mostly near V bgw under poor soaring conditions and closer to V opt in good soaring conditions.Honey buzzards most adopted a time-optimal strategy when crossing the Sahara, and at the onset of spring migration, where and when they met with the best soaring conditions. The buzzards nevertheless glided slower than V opt during most of their journeys, probably taking time to navigate, orientate and locate suitable thermals, especially in areas with poor thermal convection.Linking novel tracking techniques with optimal migration models clarifies the way birds balance different tradeoffs during migration. List of symbols and abbreviationsb Angle between heading and track direction over full climb-glide cycle. V a Gliding air speed. V ac Horizontal air speed during the climb phase. V bg Theoretical best-gliding air speed for maximizing gliding range relative to the air. V bgw Theoretical bestgliding air speed for maximizing gliding range relative to the ground. V c Climb rate. V cc Cross-country speed relative to the air. V g Gliding ground speed. V h Ground speed during the climb phase (mostly due to advection by wind). V ms Theoretical minimum-sink speed. V opt Theoretical optimal air speed for maximizing V cc . V s Sink rate. V xc Cross-country speed relative to the ground. TEA Thermal exit altitude.

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Section: Discussionmentioning

confidence: 99%

“…We used linear regression models to test for possible correlations between wind conditions and soaring conditions since thermals tend to lose vertical velocity in strong winds (Stull ). We also tested for correlation between thermal exit altitudes (TEA) and climb rates V c (Shamoun‐Baranes et al ).…”

Section: Optimal Soaring Migration Theorymentioning

confidence: 99%

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Vansteelant

Shamoun‐Baranes

McLaren

et al. 2017

Journal of Avian Biology

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“…Most previous work on flight altitude of migrating birds, especially raptors, uses radar to focus on species that use convective thermals to power flight (Kerlinger, Bingman & Able ; Kerlinger & Gauthreaux ; Kerlinger ; Leshem & Yom‐Tov ; Spaar & Bruderer ; Spaar, Liechti & Bruderer ; Shamoun‐Baranes et al . ; Dokter et al . ).…”

Section: Introductionmentioning

confidence: 99%

Topography drives migratory flight altitude of golden eagles: implications for on‐shore wind energy development

Katzner

Brandes

Miller

et al. 2012

Journal of Applied Ecology

131

143

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Summary 1.Wind power is a fast-growing industry with broad potential to impact volant wildlife. Flight altitude is a key determinant of the risk to wildlife from modern horizontal-axis wind turbines, which typically have a rotor-swept zone of 50-150 m above the ground. 2. We used altitudinal GPS data collected from golden eagles Aquila chrysaetos tracked using satellite telemetry to evaluate the potential impacts of wind turbines on eagles and other raptors along migratory routes. Eagle movements during migration were classified as local (1-5 km h À1 ) or migratory (>10 km h À1 ) and were characterized based on the type of terrain over which each bird was flying, and the bird's distance from wind resources preferred for energy development.3. Birds engaged in local movements turned more frequently and flew at lower altitude than during active migration. This flight behaviour potentially exposes them to greater risk of collision with turbines than when engaged in longer-distance movements. 4. Eagles flew at relatively lower altitude over steep slopes and cliffs (sites where orographic lift can develop) than over flats and gentle slopes (sites where thermal lift is more likely). 5. Eagles predominantly flew near to wind resources preferred by energy developers, and locally moving eagles flew closer to those wind resources with greater frequency than eagles in active migration. 6. Synthesis and applications. Our research outlines the general effects of topography on raptor flight altitude and demonstrates how topography can interact with raptor migration behaviour to drive a potential human-wildlife conflict resulting from wind energy development. Management of risk to migratory species from industrial-scale wind turbines should consider the behavioural differences between both locally moving and actively migrating individuals. Additionally, risk assessment for wind energy-wildlife interactions should incorporate the consequences of topography on the flight altitude of potentially impacted wildlife.

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“…, Shamoun‐Baranes et al . ). At night, however, in the absence of vertical mixing induced by convective thermals, the convective boundary layer collapses and the atmosphere becomes generally more stratified (Stull ).…”

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The influence of weather on the flight altitude of nocturnal migrants in mid‐latitudes

Kemp

Shamoun‐Baranes

Dokter

et al. 2013

Ibis

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By altering its flight altitude, a bird can change the atmospheric conditions it experiences during migration. Although many factors may influence a bird's choice of altitude, wind is generally accepted as being the most influential. However, the influence of wind is not clearly understood, particularly outside the trade‐wind zone, and other factors may play a role. We used operational weather radar to measure the flight altitudes of nocturnally migrating birds during spring and autumn in the Netherlands. We first assessed whether the nocturnal altitudinal distribution of proportional bird density could be explained by the vertical distribution of wind support using three different methods. We then used generalized additive models to assess which atmospheric variables, in addition to altitude, best explained variability in proportional bird density per altitudinal layer each night. Migrants generally remained at low altitudes, and flight altitude explained 52 and 73% of the observed variability in proportional bird density in spring and autumn, respectively. Overall, there were weak correlations between altitudinal distributions of wind support and proportional bird density. Improving tailwind support with height increased the probability of birds climbing to higher altitude, but when birds did fly higher than normal, they generally concentrated around the lowest altitude with acceptable wind conditions. The generalized additive model analysis also indicated an influence of temperature on flight altitudes, suggesting that birds avoided colder layers. These findings suggested that birds increased flight altitudes to seek out more supportive winds when wind conditions near the surface were prohibitive. Thus, birds did not select flight altitudes only to optimize wind support. Rather, they preferred to fly at low altitudes unless wind conditions there were unsupportive of migration. Overall, flight altitudes of birds in relation to environmental conditions appear to reflect a balance between different adaptive pressures.

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Differential Use of Thermal Convection by Soaring Birds Over Central Israel

Cited by 56 publications

References 8 publications

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Topography drives migratory flight altitude of golden eagles: implications for on‐shore wind energy development

The influence of weather on the flight altitude of nocturnal migrants in mid‐latitudes

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