Mechanics and control of perching flight

Thesis

Perching is an example of high risk, high reward behaviour executed within tight physical constraints. Despite the high risks posed by collision and stall, perching is performed routinely by most birds. However, surprisingly little is understood about the energetics and control of aerodynamic braking, nor the detail of wing and tail kinematics during perching.

In this thesis, detailed wing and tail kinematics were quantified for over 1000 indoor perching flights by Harris hawks using high-speed motion capture. Similar measurements were also taken from 15 outdoor flights by a Steppe eagle, and trajectories were tracked from wild passerines landing on a fixed or oscillating feeder. Aerodynamic braking is typically achieved in raptors through a rapid pitch-up manoeuvre executed at the end of a gliding approach. This manoeuvre produces high drag as predicted by published unsteady aerodynamic models, but unexpectedly the data shows no accompanying peak in lift. The complexity of wing-tail morphing and rotation during the rapid pitch-up manoeuvre points to the careful control of aerodynamic forces during unsteady deep stall.

A high degree of variation in the perching manoeuvre was evident in the Steppe eagle flights, likely in response to local air conditions. However the hawk flights were exceedingly consistent under the controlled experimental conditions. Different perch distances were used to induce variation in the hawks’ braking strategy. Braking was achieved in a flapping rather than gliding approach in the shortest flights, although the observed pattern of body pitch-up was consistent across flight distances. Systematic inter-individual variation was recorded in both wing motion and trajectory shape.

As a visually-guided flight behaviour, braking is controlled precisely with respect to the target such that the bird may land with low impact force and avoid injuries. Flights from the eagle and hawks show evidence for control of braking in line with the predictions of tau theory: during approach to the perch, both the rate of change in the optic flow variable tau and the rate of deceleration were constant. This was not the case for wild passerines using intermittent flight, although they might still use optic flow in feedback control.

Authors: France, L

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: flight; motion capture; eagle; landing flight; flight control; bird flight; flight kinematics; Harris hawk; wing motion; flight mechanics; morphing wings; perching
• Subjects: Birds--Flight