Indirect estimates of wing-propulsion forces in horizontally diving Atlantic puffins (Fratercula arctica L.)

Instantaneous force production in wing-propelled diving Atlantic puffins (Fratercula arctica) was investigated using four birds for which instantaneous estimates of velocity and acceleration of the body were made. The quasi-steady resultant force acting on the body in the sagittal plane was calculated using acceleration reaction coefficients, buoyancy estimates, and drag coefficients taken from the literature and calculated, using two different methods, from a video sequence of a puffin gliding (CDw = 0.021 and 0.026, respectively). The forces calculated from the motion of the body coincide well with the wing-beat cycle, with a forward- and upward-directed force produced by the wings during the downstroke and a forward- and downward-directed force produced during the upstroke. The result suggests that a thrust force may also be produced during at least the lower-stroke reversal. This means either that there may exist some undescribed propulsive mechanism, possibly caused by an interaction of the wings beneath the body, or that the body drag coefficient may be overestimated. However, the body drag coefficient calculated in the study is close to the coefficients measured on dead birds. Furthermore, I conclude that the high reduced frequency (average 0.82) suggests a non-steady-state hydrodynamic mechanism of wing-propelled diving in puffins.