Debajyoti Karmaker

Julia Groening

Ingo Schiffner

Mandyam V. Srinivasan

Abstract

This study investigates whether, and, if so, how, birds use vision to avoid a moving obstacle. Budgerigars, Melopsittacus undulatus, were filmed in 3-D using high-speed video cameras as they flew along a 25m tunnel in which a swinging pendulum, carrying a 41cm disc, presented an oscillating obstacle that moved back and forth across the bird's flight path. To our knowledge, ours is the first study to examine how birds cope with a moving obstacle. Our results reveal that birds approaching the moving pendulum ensure a collision-free passage by always aiming for the visually wider gap at the time of crossing the pendulum. Furthermore, they enhance the safety of the passage through the wider gap by increasing their flight speed slightly when the pendulum is approaching them, and decreasing it slightly when it is receding, thus ensuring that the chosen gap is as wide as possible at the time of transit. These findings provide direct evidence that Budgerigars can identify the wider gap, as well as distinguish whether this gap is currently widening or narrowing. The collision-avoidance scenario investigated here is very different from the well-studied phenomena of birds flying in a flock, or aircraft flying in a formation. There, all of the birds (or aircraft) are flying predominantly in the same direction, making imminent collisions easier to detect and prevent. This study explores a scenario in which a single bird is suddenly confronted by a moving obstacle, say, a tree branch or another bird, approaching from a completely different direction.