But the birds did not react to the imagery based upon speed. Instead, the scientists observed, they used object size in the vertical axis to figure out how far away objects were, when flying left or right -- bigger meant closer while smaller meant farther away. In the test, the birds steered toward smaller imagery and away from bigger patterns.
"When objects grow in size, it can indicate how much time there is until they collide, even without knowing the actual size of the object," said the study's lead author Roslyn Dakin, UBC postdoctoral fellow, in a statement. "Perhaps this strategy allows birds to more precisely avoid collisions over the very wide range of flight speeds they use."
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The team also found that hummingbirds gauged their altitude similarly to insects, based on vertical-axis movement. They changed their flight -- up or down -- based on whether the patterns were going up or down.
"Our results suggest that in natural settings, birds may avoid collisions by monitoring the vertical size, expansion, and relative position of obstacles," the team wrote, in a paper just published in the journal Proceedings of the National Academy of Sciences.