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Re: Avian flight stroke origin

2011/8/15 Tim Williams <tijawi@gmail.com>:
> A clarification: the opposable hallux of juvenile hoatzins is used
> principally to _clamber_ through branches, not climb up trunks.
> During branch-clambering, the hoatzin chick's foot is employed for
> grasping branches, which is the same mechanism used for perching.
Ok, although I imagine forming a pincer where the tips can sink in the
bark they may, in pinciple, be also imagined as better to climb trees
than a pamprodactyl pes, which requires gravity to sink the claws on
the substrate. The anisodactyl/zygodactil pes counts with the pincer
effect in addition to the gravity one, which may be useful on certain
occassions when the distal extremity of digit III does not point
upwards, even on tree trunks.
Besides, I forgot to note previously that the digit forming a less
than 180º opposition in climbing piciforms is not a thumb, but digit
fourth, and that because of this, and the different mechanical effects
of flexing these different digits (likely better for digit four in
climbing) it many not be extrapolated the value of different thumb
orientations for climbing on the basis of that on the other side of
the pes. More on this later.