Meeting Abstract
Optic flow can provide information about the distance between an observer and passing obstacles. It has been proposed that flying honeybees, flies, and birds use a feature of optic flow called pattern velocity to navigate narrow passageways. Although this hypothesis is well supported for bees and flies, the evidence for birds is indirect and based on their avoidance of vertical features that provide high pattern velocity. We directly tested the influence of pattern velocity on hummingbird flight trajectories by manipulating the motion of patterns on the side walls of a narrow flight tunnel traversed by Anna’s hummingbirds. We found that for both vertical stripe and dot patterns, the tested birds’ flight trajectories were not influenced by pattern velocity manipulations as predicted if they were avoiding high pattern velocities. Further experiments demonstrate that hummingbirds instead avoid features that are larger in the vertical axis, which can explain their avoidance of vertical stripes without invoking pattern velocity. For example, birds strongly deviated towards small-sized horizontal stripes when presented with large horizontal stripes on the opposite side. In contrast, when the left and right side had vertical features of the same height (but different width), birds stayed approximately centerline. We propose that visual expansion may play a key role in allowing birds to avoid obstacles in complex environments.
