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Re: Avian flight stroke origin

As Tim says, the opposed thumb in the hoatzin is instrumental for it
to climb. However, the opposed thumb of most perching birds does not
seem to be a device much related to climbing, given that most perching
birds (with this type of thumb) are poor climbers. In fact, although
there are chameleons, many climbers (overall mammalian) do not have
digits as opposed as in perching birds, in which the pollex forms a
close to 180º to pedal digit III, if the pes is posed upon a surface.
Most mammals with opposable thumbs have them at a quite lower angle,
as in our manus, in which the plane formed by the flexed pollex is
never the same formed by the other digits. If we relate arboreal
locomotor performance with the degree of opposition of the digits on
the pedes, we may infer some incomplete, mammal-like opposition is
better for tree dwelling than the more complete opposition of perching
birds. Of course, many other features seem to better help the superior
performance of most arboreal mammals, as the rotatory articulations,
as Tim indicated. Yet, it has been observed for piciforms that fingers
forming a nearer to 90º angle to the third digit are better related
with climbing habits, while those forming 180º are better for perching
(Bock & Miller, 1959; see Figs. 1 and 2). So, lack of a 180º thumb
opposition in Archaeopteryx may not imply, on itself, a better
qualifier for the hoatzin as an arboreal animal, and a lesser degree
of opposition may even represent an advantage for the former.
As an aside, for the sake of perching, which seems to be the better
thing the opposite hallux of modern birds can perform (other than
helping killing prey in raptors, or manipulating stuff in some
dexterous birds), pedes with non-opposite thumbs can perform similar
tasks, for example, in bats and swifts, which can "perch" in trees,
even if top-down.

Bock, W. J., and W. D. Miller. 1959. The scansorial foot of the
woodpeckers, with comments on the evolution of perching and climbing
feet in birds. American Museum Novitates, 1931: 1-45 (available for
free here: