I always understood from biology class that you need stereo vision to judge distances etc, but I see song-birds hopping from small twig to small twig with no problems at all.
How do birds process information relating to depth and distance with information coming only from a single source?
I suppose related to that I should ask how humans who lose an eye cope with judging depth/distance - I remember an experiement where we covered one eye and attempted to thread a needle and it was much more difficult than usual.
Is it something the brain learns to compensate for quickly, or will monocular humans always struggle?
All birds have an visual overlap area in the front of their face. Predatory birds like owls have a wide field of stereoscopic vision, prey birds like woodcocks have a much narrower field of overlap but a wider visual field. In fact, IIRC, woodcocks have their eyes set so far back on the sides of their heads they have a 360 degree field of vision, with stereo vision both in from of their beaks and a smaller field behind their heads. Cool.
When birds need to judge a distance accurately they cock their little heads to look straight at the object.
I cope fine without stereo vision… or at least not full stereo vision. It’s difficult to know how much I do or don’t have - my right eye normally seems to be more or less ignored by my brain, and the world doesn’t look noticeably different if it’s covered up. I can catch a ball, play squash, drive etc just fine.
Although stereoscopic vision is handy, it’s far from the only way to judge distance. It’s only good out to something like 20-50 feet anyhow - further away than that, you rely much more strongly on other cues.
Those other cues include the apparent size of an object (a distant object appears smaller than when it is closer), the effort required to focus on it, whether or not other objects appear obscured it (appear in front of it) or are obscured by it (appear behind it), and the “hazing” or “blueing” that occur with distance, particularly in a humid or dusty atmosphere.
Someone with just one working eye uses these other cues to judge distance, and beyond the useful range of stereoscopic vision may be just as accurate as anyone with two eyes. Ditto for birds - they get a lot of practice judging distances, so the get pretty good at it. Young birds, however, may spend several minutes looking at a distant perch, moving their heads to observe it from different angles, and do occassionally miss their target. There is some learning involved.
Like l_k, I have limited stereo vision. The only ill effects I have noticed are an utter inability to see the hidden image in those magic picture things (the field of dots that becomes a clear image when you focus just right), and the inability to hit a fastball when batting righthanded. Luckily, I’m a switch hitter so it’s not much of a problem.
There are so many clues to the distance of an object that I’d bet that most people’s brains don’t rely on stereo vision very much. It did end my dream of being a pool shark, however.