Meeting Abstract
Birds naturally perform many behaviors that require modulation of their aerodynamic force production via changes in wing kinematics. Although considerable work has been done on wingbeat kinematics used to vary flight speed, a proxy for modulating thrust, comparatively less work has been done on how birds modulate lift, other than in hummingbirds. To examine this in forward flight, we trained zebra finches (Taeniopygia guttata) to fly carrying varying amounts of weight, up to 75% body mass, which required birds to generate corresponding magnitudes of lift. We examined two types of kinematics: kinematics that could affect aerodynamic force through increasing wing velocity (amplitude, frequency, downstroke ratio); and kinematics that affect force through changing wing shape (wing folding). For the kinematics that affect wing velocity, we found that birds increase wing velocity with increased weight through increasing wingbeat amplitude but not wingbeat frequency. Although, these parameters both have an equal effect on wing velocity and therefore force production, the preference for increasing amplitude and not frequency implies an energetic difference between the two. This hypothesis was confirmed with a work loop preparation. Downstroke ratio decreased with increasing weight, indicating the birds used a shorter and faster downstroke to generate greater downstroke velocity and lift. For kinematics that affect wing shape, birds folded their wings to a greater extent during upstroke as they carried increasing weight. Upstroke naturally produces counterproductive forces, i.e., negative lift, and folding reduces the negative lift thereby increasing net lift. Overall, birds increase net lift production with kinematics that produce a combination of increasing positive lift during downstroke and decreasing negative lift during upstroke.
