Optical clues help flying birds

Australian scientists say they've discovered how birds avoid collisions with objects and each other as they perform their aeronautic maneuvers. The graceful flight of birds, even in crowded environments, is a result of their perception of something called optic flow, Partha Bhagavatula of the University of Queensland said. "Our findings show, for the first time, that birds regulate their speed and negotiate narrow gaps safely by balancing the speeds of image motion, or optic flow, that are experienced by the two eyes," Bhagavatula said in a university release. In a study, researchers trained parakeets to fly through a 23-foot corridor lined with varying combinations of thick black horizontal and vertical stripes, and then filmed their flights. They found birds flew down the center of the corridor when optic flow cues were balanced -- with identical, vertical stripes on either side of the corridor -- but flew more closely to one wall or another when the cues were unbalanced, for example, when one wall was lined with horizontal stripes and the other with vertical stripes. Also, the birds flew faster when the tunnels were lined with horizontal, rather than vertical, strips, suggesting they were using optic flow cues to regulate their flight speed. While similar flight behaviors have been demonstrated in flying insects, this is the first time the use of optic-flow signals has been demonstrated in birds, the researchers said.