Meeting Abstract
By gliding up to an elevated perch, a bird can convert some of its kinetic energy into potential energy. Besides storing energy that can be used again for take-off, this manoeuvre reduces the requirement for energetically-costly aerodynamic braking during the approach. The most efficient approach to the perch would limit the use of braking to trajectory corrections only. We investigated the constraints on the transition from powered flight to an unpowered perching manoeuvre by simulating constant-lift decelerating glide trajectories towards an elevated perch. We identified several physical constraints that limit the horizontal and vertical distance to the perch by which gliding must be initiated. The requirement to fly faster than the stall speed causes the minimum required vertical distance to increase for smaller horizontal distances. On the other hand, a larger vertical distance requires a steeper pull-up manoeuvre, which leads to two further limits constraining the maximum vertical distance to the perch: the maximum load that the wing can sustain, and the need to avoid an inverted approach. Together, these limits constrain the minimum horizontal distance that a bird needs to perform a successful perching manoeuvre. In an experimental setting, involving four Harris’ hawks, we controlled the distance between perches and found that the behaviour of these birds matched the modelled limits: when the perch distance was large, the birds flew near to the floor, with a gliding approach to the perch; for short distances, the birds flew straight to the perch and instead performed a powered braking manoeuvre. As straight, level flight would in principle work for any perching distance, the observed flexibility in strategy demonstrates an implicit awareness of the underlying physical constraints.
