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Re: DINOSAUR digest 1158



Mickey Rowe <rowe@psych.ucsb.edu> wrote:

> Probably not.  Once again (as you've inferred), the lack of
> extra-ocular muscles is the primary reason that birds have to cock
> their heads in order to get better views of objects.  To expand a
> bit...  Animals vary in the extent to which their retinas vary across

What happens when birds fly? Looking at the birds around me and the
excellent close-ups on Attneborough's Life of Birds they appear to hold
their heads staic (or at least they don't bob).

> space.  We have a very high density of cone photoreceptors at one
> point in each eye, and our cone density falls off pretty quickly as
> you move away from that point.  Some animals have no such regions of
> higher density, and hence no real reason to prefer to use one part of
> their eyes over another.  Birds typically have two regions of high
> cone density, each associated with different behaviors.  Thus they may
> alternately direct one of these "foveas" at an object and then the
> other.  Lacking an ability to rotate their eyes they must rotate their
> heads in order to accomplish this task.  Also, at least some birds
> have a gradient in cone density such that they have higher acuity on
> the top of their retinas (the part that's "looking" down) than the

Used in flight for observing the ground?

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