Re: over-achieving bird-brains

[Mike Habib <habib@jhmi.edu>]

On Jan 31, 2009, at 7:29 PM, john bois wrote:

> Avian brain evolution: new data from Palaeogene birds (Lower Eocene)  
> from England, Milner and Walsh, Zoo J. of the Linnean Soc., 2009,  
> 155, 198-219.
>
>
> ...How did this play out? The authors confront the problematic issue  
> of why only the neornithines made it through.
> They dismiss two hypotheses:
> 1. Replacement pre-K/T. Based on endocasts (and, again, comparing  
> brain and bony labyrinth structures of extant and extinct birds)  
> they find that neornithines probably did not enjoy much advantage  
> over enantiornithines in flight capability--though I would have  
> liked to see a discussion on tail length before closing this case!

I doubt that tail length would, in and of itself, be much of a  
kicker.  Tail morphology has an impact on flight dynamics, granted,  
but it isn't a simple relationship.

However, I do offer some skepticism regarding the comparison of  
"flight ability" for several reasons:

1) The authors repeatedly refer to flight as "good" or "poor", when  
such designations have little to no meaning.  Based on the context,  
the authors seem to be referencing maneuverability.  If so, they  
should simply state this - vague terminology doesn't help anyone.   
More importantly, their predictors of maneuverability are still  
somewhat questionable, and to the extent that they are able to delimit  
patterns from brain structure, they don't really tell us anything we  
didn't know.  For example, pseudodontorns are predicted to be wide- 
turning, soaring-adapted species, which we pretty much knew already  
from the skeletal morphology.  Apparently, such soaring flight is  
"poor" flight, which again, is a silly designation.

2) If we drop the whole "good versus poor" thing, we would be left  
with more specific aspects of flight to compare.  This is helpful.   
However, it also becomes readily apparent that impact of specific  
flight attributes in an extinction/replacement context are going to be  
pretty complicated.  Long-distance flyers might be able to escape  
local effects, while clades dominated by short-ranged taxa might be  
more speciose and contain critical isolates of variation.  There are  
all sorts of interactions that can go on here, and they are masked by  
the whole "good flyer"/"bad flyer" bit.

3) The authors are really working off of two new fossils.  Yes, they  
do include reference to, and information on, other taxa, but I don't  
think going back to the root node of Neornithines using a  
pseudodontorn and a phaethonid is particularly robust, at least in  
terms of reconstructing mental power and flight dynamics.

Might have other thoughts as time goes on.  I don't think the paper  
was awful, I just don't think the "flight ability" bit was very  
strong.  They would have done better to talk more narrowly about  
turning radius and perhaps average flight agility, as well.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181
habib@jhmi.edu