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

Williams, H. J., Duriez, O., Holton, M. D., Dell'Omo, G., Wilson, R. P., & Shepard, E. L. C. (2018). Vultures respond to challenges of near-ground thermal soaring by varying bank angle. Journal of Experimental Biology, 221(23): jeb174995. doi:10.1242/jeb.174995.

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https://doi.org/10.1242/jeb.174995 (Any fulltext)
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 Creators:
Williams, Hannah J.1, Author              
Duriez, Olivier, Author
Holton, Mark D., Author
Dell'Omo, Giacomo, Author
Wilson, Rory P., Author
Shepard, Emily L. C.1, Author              
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1ou_persistent22, ou_persistent22              

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 Abstract: 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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 Dates: 2018-12-03
 Publication Status: Published in print
 Pages: -
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 Table of Contents: -
 Rev. Type: -
 Identifiers: DOI: 10.1242/jeb.174995
ISSN: 0022-0949
 Degree: -

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Title: Journal of Experimental Biology
Source Genre: Journal
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Publ. Info: London : Published for the Company of Biologists Ltd. by the Cambridge University Press
Pages: - Volume / Issue: 221 (23) Sequence Number: jeb174995 Start / End Page: - Identifier: ISSN: 0022-0949
CoNE: https://pure.mpg.de/cone/journals/resource/110992357319088