Meeting Abstract

54.4  Sunday, Jan. 5 14:00  Analysis of the function and mechanics of the wing tuck manoeuvre in a steppe eagle Aquila nipalensis REYNOLDS, K.V.*; THOMAS, A.L.R.; TAYLOR, G.K.; University of Oxford, UK; University of Oxford, UK; University of Oxford, UK kate.reynolds@zoo.ox.ac.uk

Soaring birds can often be seen to pull their wings down, in a transient manoeuvre that we call a wing tuck. Here we analyse the mechanism, occurrence, and function of wing tucking in a trained, captive steppe eagle Aquila nipalensis using video and on-board inertial instrumentation. Over 3,000 tucks were identified automatically from 60 flights. Statistical analysis revealed that the rate of wing tucking is positively related to localised measures of the mean and fluctuating components of wind speed and mean flight altitude, which serve as proxies for mechanical turbulence intensity. The body motions preceding a tuck are initially reminiscent of the phugoid mode in a fixed-wing aircraft: airspeed, pitch angle, and load factor all ramp up whilst angle of attack remains constant. This seems consistent with the bird encountering a headwind gust. Immediately before the tuck movement there is an abrupt fall in angle of attack, and consequently in load factor, which suggests the bird has entered a downdraft. We propose that this loss of loading initiates the tuck manoeuvre as there is no longer sufficient lift generated to fully support the wings. Normal loading is quickly recovered as the wings re-open. We conclude that wing tucks are a gust alleviation mechanism, serving to reset the flow when the wings encounter adverse loading, and thereby allowing flight in more turbulent conditions. Turbulence is generated by thermal and mechanical processes which also produce the updrafts used by birds in soaring flight. From our data we hope to determine other soaring strategies, like the wing tuck, that birds might use to deal with, and potentially, exploit these processes to soar efficiently.