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Kinematics of the Avian Wing and Shoulder during Ascending Flapping Flight and Uphill Flap-Running

David B. Baier. Stephen M. Gatesy, Kenneth P. Dial (2013)
Three-Dimensional, High-Resolution Skeletal Kinematics of the Avian
Wing and Shoulder during Ascending Flapping Flight and Uphill
Flap-Running.  PLOS ONE 8: e63982


"Past studies have shown that birds use their wings not only for
flight, but also when ascending steep inclines.
Uphill flaprunning or wing-assisted incline running (WAIR) is used by
both flight-incapable fledglings and flight-capable
adults to retreat to an elevated refuge. Despite the broadly varying
direction of travel during WAIR, level, and descending
flight, recent studies have found that the basic wing path remains
relatively invariant with reference to gravity. If so, joints
undergo disparate motions to maintain a consistent wing path during
those specific flapping modes. The underlying
skeletal motions, however, are masked by feathers and skin. To improve
our understanding of the form-functional
relationship of the skeletal apparatus and joint morphology with a
corresponding locomotor behavior, we used XROMM (Xray
Reconstruction of Moving Morphology) to quantify 3-D skeletal
kinematics in chukars (_Alectoris chukar_) during WAIR
(ascending with legs and wings) and ascending flight (AF, ascending
with wings only) along comparable trajectories.
Evidence here from the wing joints demonstrates that the glenohumeral
joint controls the vast majority of wing
movements. More distal joints are primarily involved in modifying wing
shape. All bones are in relatively similar orientations
at the top of upstroke during both behaviors, but then diverge through
downstroke. Total excursion of the wing is much
smaller during WAIR and the tip of the manus follows a more vertical
path. The WAIR stroke appears ‘‘truncated’’ relative to
ascending flight, primarily stemming from ca. 50% reduction in humeral
depression. Additionally, the elbow and wrist
exhibit reduced ranges of angular excursions during WAIR. The
glenohumeral joint moves in a pattern congruent with
being constrained by the acrocoracohumeral ligament. Finally, we found
pronounced lateral bending of the furcula during
the wingbeat cycle during ascending flight only, though the phasic
pattern in chukars is opposite of that observed in
starlings (_Sturnus vulgaris_)."

Because it's a PLoS One paper, the main text is highly explanatory,
and the paper is replete with great figures.  These PLoS papers are a
joy to read.

One revelation is that uphill flap-running or WAIR (wing-assisted
incline running) requires extensive motion at the glenohumeral joint,
comparable to ascending flight. Although not directly addressed in
this particular study, WAIR has been proposed as an incipient flight
behavior in the ancestors of birds; this hypothesis has been advanced
in a number of publications (including in some especially high-impact
journals).  However, given the limited range of humeral elevation
inferred for non-avian maniraptorans and early birds, I very much
doubt that they were capable of WAIR.  If, as some people have
suggested, early flying theropods had a deltoideus-driven upstroke (to
compensate for limited humeral elevation), this would seem to be
incompatible with WAIR - and controlled flapping descent (CFD) too.

Although WAIR is a fascinating behavior exhibited by some modern
avians, there doesn't seem to be much to recommend it as an incipient
flight behavior in the evolution of flapping flight.  This conundrum
has been raised before.  WAIR is an "advanced" flight behavior, even
if it can be executed using incipient wings (such as present in
juvenile chukars).