'hovering' Flight of Wilson's Storm Petrel

Wilson's storm petrel (Oceanites oceanicus) characteristically feeds by ' hovering' over the water surface, but its technique for this is unlike that of other flying vertebrates. The kinematics and aerodynamics of this ' hovering' flight were investigated to determine the possible sources of lift; various nonaerodynamic sources of lift were discounted. It is suggested that the storm petrel soars into an ambient, horizontal wind, and thus is not hovering in the usual sense. Such soaring into a horizontal wind is only possible if some thrust component counteracts the bird's aerodynamic drag, and it is shown that the hydrodynamic drag of the feet through the water is adequate to balance aerodynamic drag. The bird is thus analagous to a kite, where the tension in the string counterbalances the aerodynamic drag of the kite. The movements of the storm petrel's wings during 'hovering' suggest that the bird may use the wing-flip mechanism for generation of high lift coefficients (Weis-Fogh, 1973, 1976). Such high lift coefficients are required for the bird to ' hover' under calm conditions, when ambient wind velocity is less than 5 m s". Use of the wing-flip mechanism would enable the bird to 'hover' at lower ambient wind velocities. Ground effect also contributes to the bird's ability to 'hover'. This analysis of the flight behaviour of Wilson's storm petrel indicates that its 'hovering' is an energetically inexpensive foraging stragegy which is probably available only to small, surface-feeding birds with low wingloading.