Hunting kestrels were observed to hang, almost without wing-flapping, in fixed positions over a sea dike. The height and position with respect to the dike profile, the wind direction and velocity and the percentage of hunting time without wing beating were recorded in 429 cases.
The vertical wind angle, θ, the wind speed and its horizontal direction were measured at 13 heights up to 8.8m above the windward slope, the top and the leeward slope of the dike under various wind conditions in 225 cases. These wind profile measurements were used to estimate 6 and wind speed near the hanging birds.
Kestrels hanging more than 90% of the hunting time preferred a position 6.5±1.5m (S.D.) over the windward slope with sea winds blowing at 8.7±1.5ms−1 (S.D.) perpendicular (±30°) to the longitudinal dike axis. For these birds angle θ was approximately 6–7°.
These angles are larger than expected from aerodynamic models and windtunnel measurements. The minimum gliding angle for a kestrel under steady conditions is estimated to be 5°.
Hanging kestrels save two-thirds of the energy used during normal windhovering but have to spend 1.6 times more time to catch the same number of voles.