Re: Feather Flap

[MICHAEL HABIB <habib@jhmi.edu>]

"Nevertheless, certain maniraptorans do show some traits that might be 
associated with arboreality - or at least scansoriality. There's small body 
size, for one (yeah, I know this one's a stretch). There's possible 
scansorial/arboreal traits in the manus and pes, including (for example) an 
enlarged hallux that has shifted distally on the foot compared to other 
theropods. Still not a reversed hallux, just a longer and lower one."

Which brings up the question of what characteristics we actually associate with 
arboreality in birds (and/or near-bird theropods).  The reversed, gripping 
hallux is obviously a distinctive characteristic when it appears, but I don't 
think we should be overly reliant on that character (it is also worth pointing 
out that a reversed hallux does not always indicate arboreal habits: the animal 
could also be secondarily terrestrial, as in roadrunners).

For one thing, the theropods in question may have been able to climb somewhat 
quadrapedally.  That changes the mechanics of climbing enough that the 
correlation between arboreal habits and hallux morphology may be looser than we 
expect.  In addition, while tight gripping of branches is quite helpful and 
important for modern arboreal birds, I am not convinced that it is required for 
an arboreal existence (especially in animals with powered flight or extensive 
gliding ability).

One set of characters of note in the regard are actually structural characters: 
extant arboreal birds (including modern passerines and falconimorphs, for 
example) seem to fall within a similar range for certain quantitative, 
structural characters associated with limb loading.  The most likely 
explanation is evasive flight in forested settings (and more specifically, the 
resulting accelerations and momentum changes), but that's still tentative.  Any 
other discrete qualitative or quantitative characters that are highly 
correlated with arboreality in avian lineages (other than food morphology)?

Cheers,

--Mike H.