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aerodynamics vs display in bird tails (was Re: Who says dromaeosaurs can't fly?)



From: Michael Lovejoy <michael@palaeoproductions.fsnet.co.uk>
To: DML <dinosaur@usc.edu>
Date: Wednesday, 18 September 2002 9:13
Subject: RE: Who says dromaeosaurs can't fly?


>Ronald Orenstein wrote: "all extant birds have a pygostyle NO extant bird
>has a long tail with a tuft on the end."
>
>Actually I didn't say extant birds had LONG tails with a spray of feathers,
>just that they had tails. Are you suggesting that the pygostyle didn't
>evolve from a long tail?
>
>Ronald Orenstein also asked: "How can you be sure that it didn't serve a
>display function, either in addition to or instead of an aerodnamic one?"
>
>It just seems weird to put a display structure right out there where it
>would also have an aerodynamic effect, unless as you say, it was both for
>display and aerodynamics.
>As an aside, has anyone done any work on what effect such a structure might
>have on flight?


There was some research done on some small birds near Townsville that I saw
on the TV - two years ago, I think.  I can't for the life of me remember
what the bird was, or you did the work, but I think it might have been a
small honeyeater, and the researchers were at James Cook University.  I also
have a feeling that I saw it on Quantum.

Anyway, they were looking at tail length in these birds, and measuring tail
size against mating success in the males.  Basically, males with longer
tails increased mating success, but they weren't as manoevurable as males
with shorter tails.  I think this became a measurable problem for them if
they had territories that had a larger proportion of 'structurally complex
airpsace' - i.e. bush thickets, etc.  I presume they needed the
manoevurability to forage for insects, but it may have been for predator
evasion or ecven evading rival males - I can't remember.

Since this seems to be a topic of hot dicussion on the list at the moment,
I'll try to dig up some more details (unless someone already knows the
reference - Stephan or Ron? - in which case you can save me the effort).

I think that the interesting thing was that these guys were measuring very
small differences in tail length, and had collected their data well enough
to correlate these very subtle differences with measured differences in
mating success or flight performance.  I think.  I don't remember that they
physically manipulated the tails (as some earlier studies have done - e.g.
Andersson's work in the '80s on widow birds) - if not, then that is what
would have impressed me.

And talking of 'hypothesis testing', as I was flicking through my Australian
Birds book to try and remind myself of the study species, it struck me how
many birds over here would make good models to look at the relationship
between display and aerodynamic performance in tail lengths.  There are a
few families which have lots of closely related species that vary slightly
in tail length, foraging ecology, sexual dimorphism, reproductive ecology,
etc. -  for example honeyeaters, fantails, flycatchers, and fairy-wrens.
You might predict, for example, that IF sexual selection tends towards long
tails, but selection for flight performance produces a limit to the length
of a tail, then congenerics which differ in habit preference (scrub vs
grassland, for example) should exhibit a corresponding difference in tail
length.

But, of course, that is relevant only to Austalo-Papuan basal
passeriniformes.  To extend any conclusions from this field ecology to other
types of flyers you would have to construct a thorough biomechanical model
of your little brown job as a flying organism, and make careful comparisons
between that and a similarly thorough model for your putative flying fossil
animal.  Comparisons are only really meaningful if they are made between
organisms.  Comparing the tail of a _Rhamphorhynchus_ with that of a wagtail
isn't likely to offer any quality insight, because you're ignoring the
myriad other differences between the two animals.  For example, it seems to
me (with my admittedly limited understanding of aerodyamics) that the reason
for the long tail of rhamphorhyncids is something to do with the fact that
the other end is full of teeth, which are heavy.  Okay, that's probably
wrong 'cos I don't understand it properly, or if it's right then I'm sure I
read it in Wellenhofer or somewhere, but the point is that flying is a
property of the whole animal, not a part of it, and you need to understand
the animal as a whole before you can compare it with different flying
models.

Much of the discussion on this list seems to be good at taking the animal to
bits, but a little less disciplined about putting it back togther again.  It
is my belief (there's that word again!) that this is symptomatic of much
thinking in science these days - the last three decades have seen an
increasing obsession with reductionist logic in many disciplines, without
the realisation that reductionsim is fine if you remember to put the pieces
of your analysis back together again.  As a result the concept of the
organism has almost disappeared from meaningful biological discussion.  In
veterbrate palaeontolgy today, this manifests itself mainly in two forms;
1. endless cladistic analyses which are held to be results in themsleves
(rather than as the first stage in formulating phylogentic hypotheses).  The
organism is chopped into smaller and smaller pieces with scant regard for
how those peices actually operate and interact in the living, coherant
animal.  Perversely, less understanding (more chopping) is taken to signify
more data (wow, this character set's got a thousand characters!).  Yes, in
response to an earlier comment (I think it was John Conway's) this does
constitute a 'philosophical' difficulty that I have with the cladistic
mehtodology.  Or, perhaps to be fair to some of those who do use cladistics
but try to exercise some judgement during the process based upon their own
experience with the organisms, perhaps it is just a practical difficulty
with the method - but the basic philosophy of the method does seem to
encourage people to fall into this trap.

2. a parade of dubious analyses of 'functional morphology' (whatever that
means).


Personally, I blame Richard Dawkins, but no doubt he wasn't the first.
'there is no such thing as the organism, there is only the genes...' to
paraphrase te man.  Reminds me of Maggie Thatcher's quip - "There's no such
thing as society, only individuals".  Even if he wasn't the first to espouse
the power of reductionist logic, Dawkins was certainly very vocal about it -
not just about the power of reductionism, but in his dismissal of those
(like Gould) who dared suggest that there is more to a meaningful
understanding of biology than its dismemberment into constituent parts.

Oh well, this has turned from an attempt to inform into a rant. But then, I
still think (believe?) that this is as much a part of the scientific process
as hypothesis testing.  It's just a lot easier to do.

I'll see what I can dig up on the JCU website about the birds' tails.

Cheers
Colin

"If the vertebrate fossil record of Australia tells us anything, it is this;
dinosaurs, bad; plesiosaurs, good."

Colin McHenry
56 Gaskill St
CANOWINDRA,  NSW 2804,  Australia
Ph: +61 2 6344 1009
Mobile phone: 0428 131 858
email: cmchenry@westserv.net.au