Effect of optic flow on flying birds is inhibited by feature size (#396)
Flying animals can monitor the apparent motion of retinal images to gauge distance to surrounding objects in flight. When navigating narrow passageways, bees and flies choose flight trajectories that balance the velocity of image motion on their left and right sides. We tested the prediction that birds use a similar mechanism to navigate by removing visual features (vertical stripes) that provide optic flow-based cues from either the left or right side of a narrow flight tunnel traversed by Anna’s hummingbirds, presenting horizontal stripes on that side instead. The influence of this treatment depended on feature size: birds were most strongly influenced by the asymmetrical pattern when stripes were intermediate-sized, and less affected by smaller- and larger-sized features. Results of an additional experiment suggest that horizontal features above a critical size may inhibit the effect of unbalanced image velocity. We discuss how this inhibition may allow birds to navigate complex environments that are often characterized by optic flow asymmetries.