Landing on a swinging perch: peach-faced lovebirds prefer extremes.

Abstract

Birds frequently must land (safely) on moving branches, and seemingly accomplish this with acrobatic precision. To examine how birds target and land on moving supports, we investigated how lovebirds approach and land on a swinging perch, driven at three sinusoidal frequencies. Flight kinematics were recorded, together with landing forces and pitch torque via a perch-mounted sensor. In support of our hypothesis for stable landings, lovebirds timed half their landings (51.3%) when the perch was approaching either extreme of motion near zero velocity, exhibiting a robust bimodal strategy for landing-phase timing. Horizontal landing forces exceeded vertical forces across all landing conditions, reflecting the shallow flight trajectory (-13.2 deg relative to horizontal) lovebirds adopted to decelerate and land. A uniform body pitch angle (81.9±0.46 deg mean±s.e.m.) characterized landing across all conditions, with lovebirds using the horizontal perch reaction force to assist in braking when landing. Body pitch after landing was not well correlated and was generally opposite to the initial direction and magnitude of landing pitch torque. Flexion of the bird's hindlimb joints at landing reduced landing torque by aligning the bird's center of mass trajectory more closely to the perch. Landing pitch torque and body pitch rotation increased uniformly in response to increased perch swing frequency. In contrast to landing forces, pitch torque varied irregularly across landing conditions. Our results indicate that lovebirds regulate their approach trajectory and velocity to time the phase of landing to a moving perch, providing insight for designing biologically inspired unmanned aerial vehicles capable of landing on moving targets.