dgraham@kean.ucs.mun.ca (David Graham) (08/06/90)
In article <4082@trantor.harris-atd.com>, sonny@charybdis.harris-atd.com (Bob Davis) writes: > In article <63474@oliveb.atc.olivetti.com> mjm@oliven.olivetti.com (Michael Mammoser) writes: >> >>This difference (parallax) is the way that we make accurate >>judgements of the distance that the object is at. Birds with >>monocular vision are viewing a distant object with only one eye. >>They bob their heads back and forth to shift the position of >>their eye in order to provide this parallax. Sounds logical anyway. >> >>Mike >> > NO, NO, NO. As another poster to this thread has already > accurately observed, THE HEAD IS RENDERED ABSOLUTELY IMMOBILE > RELATIVE TO THE EARTH by the bird's "head bobbing" motion as it walks. > Look at it closely. I often have marvelled at just how totally motionless the > bird is able to hold its head as it walks. What an effective biological servo > control system this is! > I like the theory recently advanced here that it's a question of saturation vs refreshing of receptors in order to provide optimum resolution of incoming predators. But what about raptors? Perched owls and hawks will bob their heads from side to side, and tilt them as well. I've read that this is to provide best parallax prey in their visual field, but now I'm wondering if receptor saturation might not have something to do with that too (only in reverse, if you follow me). Anyone have any ideas? Sam? Kehaar? > ______________________________________________________________________________ > Bob Davis \\ INTERNET : sonny@trantor.harris-atd.com | _ _ | *************************************************************************** David Graham dgraham@kean.ucs.mun.ca ***************************************************************************
rsl@cbnewsm.att.com (randolph.little) (08/07/90)
David Graham writes: >I like the theory recently advanced here that it's a question of >saturation vs refreshing of receptors in order to provide optimum >resolution of incoming predators. But what about raptors? .... Recall that hawks and owls are exceptional among birds in that they DO have binocular vision, giving them the opportunity to use parallax clues to perceive depth. David's point is good. Let's not rule out either receptor refreshment or depth enhancement as "causes" for head bobbing in perched raptors. Both factors may apply. Randolph S. Ltle (randolph.little\@att.com) #UUCP: bonasa!rsl or mtuxo!rsl or attmail!rsl #DDD: 908-658-2640 #USMail: AT&T C02-34D64 55 Corporate Drive Bridgewater, NJ 08807-6991
jbm@safn2.UUCP (John McClatchey) (08/07/90)
In article 1512 David Graham writes: > I like the theory recently advanced here that it's a question of > saturation vs refreshing of receptors in order to provide optimum > resolution of incoming predators. But what about raptors? Perched owls > and hawks will bob their heads from side to side, and tilt them as > well. I've read that this is to provide best parallax prey in their > visual field, but now I'm wondering if receptor saturation might not > have something to do with that too (only in reverse, if you follow > me). Anyone have any ideas? Sam? Kehaar? > If some birds use head movements to improve vision, I think owls bob their heads to improve prey location by sound. Owls have true binocular vision don't they? Also, birds of prey are supposed to have such outstanding vision, does this mean that head bobbing is unnecessary for them? Perhaps head bobbing in raptors is always for hearing. If you have a dog, watch his head movements in reaction to a strange sound; it's like an owl.