bird movements

["Mickey Rowe;893-2446" <mrowe@lifesci.ucsb.edu>]

Jura <pristichampsus@yahoo.com> (a.k.a. Jason Bourke) wrote:

> Well, for one, birds locomote in a manner completely
> different from those of dinosaurs (classic dinosaurs).
> I'm not talking about flying either. No dinosaur has
> been shown to be a tibia based walker. There was an
> argument in favour of _Caudipteryx_ doing that (at
> least a little bit), but this seems to have been based
> on bad data. Dinosaurs probably didn't see like birds
> either; with relatively fixed eyes that require head
> bobbing in order to walk effectively. 

As in other examples, your attempt to use this particular character
state to define your group backfires.  As with a lot of the other
examples that have been raised and shot down, this example is based on
ignorance (not so much yours as general).  A few years back, the
subject of head bobbing came up here, and I did some digging with the
intent of commenting on it.  I never managed to find the time (and I
still don't have it now), but I can share a few relevant bits.  It is
common lore that bird's eyes are fixed within their orbits.  I've
stated that "fact" myself in past messages on this list.  However,
it's not generally true.  It may well be out and out false.

The relative fixity of the eyes is one of the reasons cited for
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?

To get back to the subject of "ignorance", birds don't generally have
immovable eyes.  There are actually people who study eye movements in
birds, but not many.  Birds generally do have a vestibulo-ocular
reflex (a counter-rotation of the eyes within their orbits in response
to rotations of the head).  And at least some have a pretty serious
ability to move their eyes both reflexively and voluntarily.  To
address both points (general ignorance and demonstrable facts), find
this reference:

Martin, G. R. (1986).  "The eye of a passeriform bird, the European
     starling (Sturnus vulgaris): eye movement amplitude, visual
     fields and schematic optics", J. Comp. Physiol. A, 159:545-557.

which contains this statement:

     there have been no descriptions of the optical properties, visual
     fields or eye movements in any passeriform eye.

If you do a cited reference search on that, you'll find that only
about five of the fifty papers address the subject of eye movements in
other birds.  In those that do look at eye movements (this may not be
exhaustive because I haven't read all of the papers), you'll find that
there are substantial eye movement capabilities in herons, zebra
finches, and albatross.  I didn't see any studies (other than
references to older studies on owls) indicating bird eye movement
capabilities were examined and found to be severely restricted.  The
abilities of these animals to move their eyes is limited relative to
ours (we can rotate our eyes about 90 degrees vs. around 30 to 40 for
this small subset of birds), but (and I invite others to look into
this) I think that says more about primate eye movement capabilities
as unusually extensive rather than birds as unusually restricted.  I'm
hoping Kent Stevens will read this and be moved to comment...

So, the idea that birds have fixed eyes isn't terribly well rooted in
data.  Data, such as they are, suggest the idea is wrong.  In any
case, it is certainly wrong in particular cases.  Perhaps Jura wants
to argue that the word "bird" be restricted to the group that most of
us refer to as "owls" :-)

-- 
Mickey P. Rowe     (mrowe@lifesci.ucsb.edu)