arf@ddsw1.MCS.COM (Jack Schmidling) (07/28/90)
Subject: Re: HEAD JERKING OF WALKING BIRDS I have a problem with Pnews here in that it makes it look like I want responses in email when I really want to discuss things on the net. The following is typical of the responses I got to the question I posed.... >You realize, of course, that this is a violation of Federal law. ............. >If it is one of the species you can legally possess, let me know, and I will comment on your behavioral questions. ............ I fully accept the criminal nature of the act which I implicated myself in, but having watched a kestral snatch the first two fledglings off the nest in my bathoom window, I was in no mood to concern myself with the letter of the law. The most amusing response (excerpt above) was from someone who refused to even discuss the issue if the bird in question was a mourning dove. ............ One responder suggested that the behavior was an adjustment for continually changing focal distance. This seems like a reasonable answer but I can't help but wonder why these birds have such a limited depth of field. One assumes that it must be on the order of 1/2 inch because that amount of head movement seems to put things right again. I suppose the answer lies in what fantastic things they must be able to see in that very restricted range. A few more experiments with the bird at arms length, seem to support the focus theory. If I was very careful to make sure he could not see me, he did seem to jerk his head no matter which way he was facing. ................. On a slightly different point, is there really any way of knowing what a bird is looking at? For example..... He will suddenly cock his head and look straight down at the floor and........... However, someone on the opposite side of the bird would swear that he was looking straight up in the air. ...... just heard the familiar wing beat and find Charlie in the bucket of sunflower hearts on the back porch. For the past five days, he has left around dark and returns late in the morning to "visit". Oh! Oh! the doorbell.... must be the bird police. Run Charlie! ............... If you insist on sending email, please send to arf@gagme NOT to the address in this header.
john@nmt.edu (John Shipman) (07/29/90)
Jack Schmidling (arf@gagme) writes: +-- | The following is typical of the responses I got to the | question I posed.... | | >You realize, of course, that this is a violation of Federal law. +-- The federal Migratory Bird Treaty prohibits any individuals from possessing wild native birds without a permit. Introduced birds such as Rock Doves (park pigeons), European Starling, Budgerigar, etc., are not included in this law. (By the way, starlings make terrific cagebirds.) +-- | I fully accept the criminal nature of the act which I | implicated myself in, but having watched a kestral snatch | the first two fledglings off the nest in my bathoom window, | I was in no mood to concern myself with the letter of the | law. +-- Typical prejudice against birds of prey---the kestrel was probably feeding young! Raptors have to eat too. No smiley here---I'm not shy about being a predator fan; a bumper sticker on my car reads, ``Save the Great White Shark.'' -- John Shipman/Zoological Data Processing/Socorro, NM/john@jupiter.nmt.edu ``Let's go outside and commiserate with nature.'' --Dave Farber
afsipmh@cid.aes.doe.CA (Patrick Hertel) (07/30/90)
In article <1990Jul28.033019.5059@ddsw1.MCS.COM> arf@ddsw1.MCS.COM (Jack Schmidling) writes: > > > Subject: Re: HEAD JERKING OF WALKING BIRDS > > I believe that the head jerking is due to the fact that a bird's eye is fixed and therefore it has to move its head to "refresh" its receptors. It would explain why the head jerk is not necessary when the bird is carried and looking off to the side. -- Pat Hertel Canadian Meteorological Centre Analyst/Programmer 2121 N. Service Rd. phertel@cmc.aes.doe.ca Dorval,Quebec Environment Canada CANADA H9P1J3
chrish@videovax.tv.tek.com (Chris Hawes) (08/01/90)
> I believe that the head jerking is due to the fact that a bird's eye is >fixed and therefore it has to move its head to "refresh" its receptors. >It would explain why the head jerk is not necessary when the bird is carried >and looking off to the side. >-- >Pat Hertel Canadian Meteorological Centre I'll buy that. Pat, if that's "known" fact, it sure sounds like it. Thanks for the best answer I've read so far on this question. Does anyone out there know about the difference in these walking bird eye receptors as compared to non-walking bird eye receptors? chris at Tektronix TV Meas. Sys. "... We gotta move these color TV's" Dire Straights
keithl@loop.uucp (Keith Lofstrom;;;646-6296) (08/02/90)
I got here late; sorry if this has already been suggested: Birds have small brains, and can't distinguish between their own motion and the motion of the predators they are trying to avoid. If you watch the head closely, it holds still while the bird's body continues forward, then jerks forward to a new fixed position. This feeds a series of stationary images to the bird's brain, and makes moving objects in the visual field more obvious. Why doesn't a bird's head move when riding on a shoulder? Birds didn't evolve to ride on shoulders; the built-in reflexes are wrong. -- Keith Lofstrom keithl@loop.uucp ...!sun!nosun!loop!keithl (503)628-3645 KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon" Design Contracting in Bipolar and CMOS - Analog, Digital, and Power ICs
mjm@oliven.olivetti.com (Michael Mammoser) (08/06/90)
In article <1990Jul28.033019.5059@ddsw1.MCS.COM>, arf@ddsw1.MCS.COM (Jack Schmidling) writes: > > I fully accept the criminal nature of the act which I > implicated myself in, but having watched a kestral snatch > the first two fledglings off the nest in my bathoom window, > I was in no mood to concern myself with the letter of the > law. What was it that you thought you were accomplishing? It seems that there are two possible consequences of your action. You deprive a kestrel of a meal and: (a) the kestrel, not being able to nourish itself, dies. Now you have a dead kestrel rather than a dead dove. Do you place an intrinsically higher value on doves than kestrels? (b) the kestrel goes off and takes some other bird. Now you have some other bird dead rather than a dead dove. Are doves worth more than some other species? What if the other bird taken was also a dove? Just what were you accomplishing? > One responder suggested that the behavior was an adjustment > for continually changing focal distance. This seems like a > reasonable answer but I can't help but wonder why these > birds have such a limited depth of field. One assumes that > it must be on the order of 1/2 inch because that amount of > head movement seems to put things right again. One of the best theories that I've heard is that the bird does this to provide the parallax necessary to make distance judgements. Animals with binocular vision, such as people, view objects with both eyes at the same time. Since the eyes are separated by some distance, the position of the object with respect to the distant background is different in each eye. 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