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Re: bird movements

either; with relatively fixed eyes that require head
bobbing in order to walk effectively.


head-bobbing among walking birds.  It *is* likely true that
head-bobbing evolved to stabilize the image projected onto the
animals' retinas, but casual observation shows that Jura's general
statement ("requires head bobbing") is false.  Birds don't all bob
their heads.  Some don't really walk.  On the ground they hop.  Some
(like loons) can't walk.  Are these animals no longer birds?


I'm hoping Kent Stevens will read this and be moved to comment...

Mickey is correct in stating that the repetitive head movements that many birds perform while walking are likely related to stabilizing the retinal image during locomotion. Rather than head "bobbing" (as in a bobbing cork), the movement is more a matter of head "darting". The bird rapidly extends the head forward, while maintaining eye height above ground level, to a point where the body will be after a step or two. The head darts forward and the body catches up. I have videotaped very slow, stealthy body movements (e.g. of cranes) where the head remains rock solid in space while the body steps slowly underneath, clearly the head movements are coordinated to counteract the displacement due to the body, not an inadvertent consequence of the body bobbing the head about during locomotion. Also, watch a bird on a swaying branch, or perched on someone's moving arm. The head will tend to stay immobilized in space while the body moves about.

Regarding eye movements, birds do not engage in as frequent coordinated (conjunctive, or "yoked") saccadic eye movements as, say, primates. Many raptorial birds are bifoveate, such as hawks, and have widely divergent optic axes. Thus they can only scrutinize an object of interest with one eye at a time. Hence one may observe them shifting between fixating binocularly while pointing the beak directly at the target and fixating monocularly wherein the beak is pointed about 45 degrees to the side to bring the area centralis to bear upon the given object. That is not to say that birds (and some reptiles) do not saccade as Mickey noted. We do have the edge on them regarding smooth saccades, or pursuit eye movements, where both eyes are continuously trained upon a given moving target point. But then, some birds, such as starlings, can dramatically shift the amount of binocular overlap, from a verge angle that provides substantial overlap for binocular vision, to divergent eyes that provide virtually 360 degree coverage. Among primates, the closest to matching this capability was achieved by the late, memorable, Marty Feldman.

Kent
http://www.cs.uoregon.edu/~kent