Debajyoti Karmaker

Mandyam V. Srinivasan

Abstract

Birds are agile fliers, that can move rapidly and safely through dense forests and complex environments. We have investigated how birds avoid mid-air collisions while traversing a stationary obstacle, using high-speed stereo video cameras to reconstruct their flight trajectories. Analysis of these trajectories reveals that, when passing the obstacle from above, the birds often use a 'flap-bounding' mode of flight to scale the obstacle. Quantitative modeling indicates that the avoidance trajectory can be characterised accurately by an initial phase in which the bird applies a constant vertical acceleration to gain height, and a subsequent ballistic phase in which the bird closes its wings and passes over the obstacle. The timing and strength of the acceleration phase are evidently tuned precisely to ensure that the maximum altitude is attained at the point of crossing the obstacle, and that a safe, injury-free clearance is achieved. Our findings suggest that the flap-bounding mode of flight, commonly used by many birds for energy-efficient cruising, is recruited by Budgerigars for an additional purpose, namely, obstacle avoidance.