Peer review of: Birds repurpose the role of drag and lift to take off and land (2019)

Reviewers' Comments:  
 
Reviewer #1:  
Remarks to the Author:  
L29: In static wings - ignores the energy required to overcome inertia in flapping wings.  
Legend, Figure 1: The figure is pretty small, so it would help if was more clearly stated. The blue L is 
total (to me, “net” implies something has been subtracted total lift) lift; the dashed vertical 
component (grey) opposes W, and the solid grey horizontal component T opposes the dashed red 
horizontal component of total drag D.  
L107: Given that this reasonable assumption is key to the inferences made, a bit more on the reasons 
would be welcome. For example, how might the substantial portion of the lift generated by leading 
edge vorticity affect this assumption? Further, induced drag is a pressure force; profile drag on the 
wing is frictional. For all the tremendous quantification here, “predominantly” is a little unsatisfying.  
L301: Was maximizing lift to drag during slow flight a paradigm? For most species, they spend so little 
time in slow flight that efficiency is irrelevant. They must simply be effective, by whatever means. This 
study does a nice job describing magnitude of the available means, but I’m not sure that for birds 
anyone ever reasonably thought that L/D was that critical.  
L313: “….[increased] in birds..” “Elevated” denotes position; not to be confused with wing position.  
L326, L335 “….leveraged…” I find the overuse of this word, away from its physical definition, 
unsettling; because the authors are here writing of Newtonian physics, its use at least unsettling; at 
worst, confusing. “employed” or your original “repurposed”.  
A terrific and thorough quantification of slow flight. The integration of the perch data with the body 
impulse is a particularly nice and understated touch.  
 
 
 
Reviewer #2:  
Remarks to the Author:  
The authors investigated how birds use the lift and drag reoriented by adjusting the stroke plane angle 
for their take-off and landing. They utilized the DLT-based high-speed photogrammetry synchronized 
with the aerodynamic force platform to measure the wing kinematics and the aerodynamic force 
simultaneously. The aerodynamic forces in the global frame were decomposed into the forces with 
respect to the wing motion that is the rigorous definition of the lift and drag. Through the analyses, 
the authors discovered that the lift can be used for the thrust or braking force as well as the weight 
support, and the drag can be used for the weight support as well as the braking force.  
 
The work is based on the methods established well by the authors and is undoubtedly of high quality, 
but I have a concern over the novelty of the study. The decomposition into the lift and drag is based 
on the wing motion. The quantitative estimate of force and power is the strength of the study, but, 
without them, we can predict the role of lift and drag in global frame and power by just looking at the 
wing motion and the angle of attack. Therefore, the discussion on the direction of lift and drag in the 
earlier studies (such as refs. 11 and 15) are valid as far as the definitions of the lift and drag are valid. 
It is not clear to me if the quantitative measurement performed in this study offer new insight into the 
bird flight. At least, I think that the utility of the drag shown in this study is not “surprising” since it 
has been suggested in previous studies.  
 
Another (minor) concern is the estimate of the power. I guess that the aerodynamic power does not 
contain the power to rotate the wings? The effect of the wing rotation (around the spanwise axis) may 
be small, but the negative power during the stroke reversal may be due to the lack of the rotational 
power in addition to the “local flow field during wing-wake interactions.” The power in this study is 
based on the wing motion and aerodynamic forces, but it does not contain the power to overcome the 

wing inertia. It is difficult to estimate the inertial power accurately (since it requires the detailed wing 
shape), but a simple analysis to compare the inertial power with the aerodynamic power would be 
helpful to estimate the total power consumption in bird flight.