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Re: Brooding over dinos



>    I can't allow the first sentence above to go unchallenged.  As Mark Orsen
>and I have learned in putting together our Dinofest 98 lecture, the
literature
>abounds with photos and life histories of birds that use their wing feathers
>in brooding.  

I won't quarrel with this - you have obviously researched the point more
thoroughly than I - but I would still argue that it is far from being a
primary
brooding technique (I was thinking primarily of eggs - I suspect that the
wings
are used more often to shelter young from heat or rain) and that I cannot
offhand think of an example of a specific adaptation (such as the brood patch
or the water-holding feathers of sandgrouse) in the wing that is
brooding-related.

>    Regarding display: personally, this is my most un-favorite explanation
for
>evolution of anything.  It seems to me, that with a few exceptions, what
birds
>are displaying, when they display, is their fitness to raise a brood or to
>kick butt (same as raise a brood).  Display only makes sense to me if it
shows
>off some other quality that is the REAL reason for displaying in the first
>place (bright plumage = health; unbroken wing/tail feathers = ability to
>forage, ability to shelter young, ability to fight).  Display for its own
sake
>seems very dubious to me as an ORIGIN to anything.

Yes, of course that is a widely-accepted function of display, and there are a
number of studies showing that there is a link between the size or
condition of
display plumes and things like health, parasite load etc.  But that does not
alter the point that the specific size, colour or shape of the plumes in
question serve directly as signals to the opposite sex (usually the female)
that she uses as a guide in mate selection, or as a signal to rival males in
the process of establishing or defending territory.  This has been shown by
experiments on African weaver-finches in which tail-plumes were artificially
shortened or lengthened - the potential mate or rival responds to the display
signal even though the health or abilities of the altered bird remain
otherwise
unchanged.  In a well-known example, Red-winged Blackbirds with their red
wing-epaulettes painted over lost their territories almost immediately. 

Thus I think it is perfectly accurate to argue that the shape, size or colour
of such plumes evolved for display - that is, as a particular type of
signalling device. What precisely is being signalled is not relevant to this
point (and it is highly unlikely that the objects of the signal are aware of
it).  Certainly whenever exaggerated plumage is known in a bird whose
behaviour
has not been studied display is put forward as a reasonable explanation of the
exaggeration, and study usually confirms this.  In fossils, of course,
confirmation is impossible, but I would certainly not feel I was going out
on a
limb to suggest display as the function of the elongated tail plumes in some
Confuciusornis specimens.  I think it would also be reasonable to postulate
display (or signalling of some sort) as the function of the tail fan in
Caudipteryx; you might suggest it could have been used as a sort of parasol,
but I can't think of a single living bird that uses its tail in this way while
I can think of plenty that use tail plumes in display.

The point is that I think there is plenty of evidence in living birds that the
value of display has been a driving selective force in the evolution of
feather
shape and form, while (except for the brood patch and the sandgrouse breast
feathering mentioned above) brooding has not.  That does not, of course, prove
that the opposite might have been the case during the early stages of feather
evolution, but it is suggestive to me at least - and of course the two
functions are not mutually exclusive.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net